Tension Headache (acetaminophen/caffeine)

To minimize risk of overdosage, recommended age-appropriate daily dosages of acetaminophen should not be exceeded.

Some clinicians suggest that when used as a mild CNS stimulant to overcome fatigue, oral doses of 100-200 mg of anhydrous caffeine are required. The manufacturers state that adults and children 12 years of age or older may receive a dosage of 100-200 mg no more frequently than every 3-4 hours.

For the treatment of apnea of prematurity, commercially available caffeine citrate injection in a loading dose of 20 mg/kg (10 mg/kg when expressed in terms of anhydrous caffeine) is administered by slow IV infusion (i.e., over 30 minutes) using a syringe infusion pump. Beginning 24 hours after the loading dose, maintenance doses of caffeine citrate of 5 mg/kg (2.5 mg/kg when expressed as anhydrous caffeine) may be administered every 24 hours, either orally or via slow IV infusion (i.e., over 10 minutes) using a syringe infusion pump. The manufacturer states that the safety and efficacy of dosing periods exceeding 10-12 days have not been established. Other dosing regimens+ for the treatment of apnea of prematurity have used caffeine doses (in terms of anhydrous caffeine) of 5-10 mg/kg, given IV, IM, or orally as a loading dose, and followed by 2.5-5 mg/kg, given IV, IM, or orally once daily. Maintenance dosage has been adjusted according to the patient's response and tolerance and plasma caffeine concentrations.

When caffeine citrate is used for the treatment of apnea of prematurity in infants with hepatic or renal impairment, serum concentrations of caffeine should be monitored and dosage adjusted to avoid toxicity.

Analeptic use of caffeine is strongly discouraged by most clinicians. However, the manufacturers of caffeine and sodium benzoate injection recommend IM, or in emergency respiratory failure, IV injection of 500 mg of the drug (about 250 mg of anhydrous caffeine) or a maximum single dose of 1 g (about 500 mg of anhydrous caffeine) for the treatment of respiratory depression associated with overdosage of CNS depressants, including opiate analgesics and alcohol, and with electric shock.

Some clinicians recommend that when caffeine and sodium benzoate injection is used in children for CNS stimulation+, an IM, IV, or subcutaneous dose of 8 mg/kg (about 4 mg of anhydrous caffeine per kg) (not to exceed 500 mg) or 250 mg/m2 (about 125 mg of anhydrous caffeine per m2) be given up to every 4 hours if necessary.

Acetaminophen injection is administered by IV infusion over 15 minutes.

Acetaminophen is also commercially available in fixed-combination with ibuprofen (Combogesic(R)IV) for IV administration. See the full prescribing information for administration of this combination product.

Several commercially available fixed-combination prescription preparations containing acetaminophen are also available for oral use. See the full prescribing information for administration of these combination products.

Acetaminophen is commercially available in various oral and rectal over-the-counter (OTC) preparations as a single ingredient or in combination with other analgesics (e.g., ibuprofen), antihistamines, or decongestants. See the FDA Orange Book and the manufacturer's Drug Facts for further information.

Caffeine may be administered orally. Caffeine citrate is administered orally or by slow IV infusion using a syringe infusion pump. Caffeine and sodium benzoate injection may be administered by IM or slow IV injection; the drug has also been administered subcutaneously.

The preservative-free commercially available injection is for single use only, and any unused portion should be discarded. It is important that such oral solution be measured accurately (e.g., using a 1-mL or other appropriate syringe).

No dosing information is available for this tab.

No dosing information is available for this tab.

There is no enhanced messaging for this drug.

Contraindicated Drug Combination: This drug combination is contraindicated and generally should not be dispensed or administered to the same patient.

Cyp1a2 Inhibitors/Fezolinetant

SEVERITY LEVEL: 1-Contraindicated Drug Combination: This drug combination is contraindicated and generally should not be dispensed or administered to the same patient.

MECHANISM OF ACTION: Inhibitors of CYP1A2 may inhibit the metabolism of fezolinetant.(1-4)

CLINICAL EFFECTS: Concurrent use of a CYP1A2 inhibitor may increase levels of and adverse effects from fezolinetant.(1-4)

PREDISPOSING FACTORS: None determined.

PATIENT MANAGEMENT: Recommendations for concurrent use of fezolinetant with CYP1A2 inhibitors differ in different regions. The US manufacturer of fezolinetant states that concurrent use with strong, moderate, and weak CYP1A2 inhibitors is contraindicated.(1) The Australian, Canadian, and UK manufacturers of fezolinetant state that concurrent use with strong and moderate CYP1A2 inhibitors is contraindicated, while weak CYP1A2 inhibitors are not predicted to cause clinically relevant changes in fezolinetant exposure.(2-4)

DISCUSSION: In a study, fluvoxamine, a strong CYP1A2 inhibitor, increased fezolinetant maximum concentration (Cmax) and area-under-curve (AUC) by 80% and 840%, respectively. Mexiletine (400 mg every 8 hours), a moderate CYP1A2 inhibitor, increased fezolinetant Cmax and AUC by 40% and 360%, respectively. Cimetidine (300 mg every 6 hours), a weak CYP1A2 inhibitor, increased fezolinetant Cmax and AUC by 30% and 100%, respectively.(1) Strong CYP1A2 inhibitors linked to this monograph include angelica root, ciprofloxacin, enasidenib, enoxacin, fluvoxamine, and rofecoxib. Moderate CYP1A2 inhibitors linked to this monograph include capmatinib, centanafadine, dipyrone, fexinidazole, genistein, hormonal contraceptives, methoxsalen, mexiletine, osilodrostat, phenylpropanolamine, pipemidic acid, rucaparib, troleandomycin, vemurafenib, and viloxazine. Weak CYP1A2 inhibitors linked to this monograph include acyclovir, allopurinol, artemisinin, belumosudil, caffeine, cannabidiol, cimetidine, curcumin, dan-shen, deferasirox, disulfiram, Echinacea, famotidine, ginseng, norfloxacin, obeticholic acid, parsley, piperine, propafenone, propranolol, ribociclib, simeprevir, thiabendazole, ticlopidine, triclabendazole, valacyclovir, verapamil, and zileuton.(5-7)

Xanthine Derivatives/Adenosine; Hexobendine; Regadenoson

SEVERITY LEVEL: 1-Contraindicated Drug Combination: This drug combination is contraindicated and generally should not be dispensed or administered to the same patient.

MECHANISM OF ACTION: Xanthine derivatives may antagonize the effects of endogenous(1) and exogenous adenosine,(2,3) regadenoson,(4) and hexobendine.(5)

CLINICAL EFFECTS: Concurrent use of a xanthine derivative use may result in decreased effectiveness of adenosine, hexobendine and regadenoson. Aminophylline may increase the risk of adenosine-induced seizures.(3)

PREDISPOSING FACTORS: None determined.

PATIENT MANAGEMENT: Patients receiving concurrent therapy with adenosine and a xanthine derivative should be monitored for decreased effectiveness of adenosine. The dosage of adenosine may need to be increased. Whenever possible, withhold xanthine derivatives for 5 half-lives prior to using adenosine in cardiac stress tests.(6) Methylxanthines should not be used to reverse the effects of adenosine in patients who experience adenosine-induced seizures.(3) Concurrent therapy with hexobendine and a xanthine oxidase derivative should also be monitored for decreased effectiveness of hexobendine.(5) The US manufacturer of regadenoson recommends that patients avoid methylxanthines (e.g. caffeine, pentoxifylline, and theophylline) for 12 hours prior to regadenoson administration. Aminophylline may be used to attenuate severe and/or persistent adverse reactions to regadenoson.(4)

DISCUSSION: In a study in six healthy subjects, theophylline significantly reduced the heart-rate response to adenosine. In addition, theophylline reduced the amount of abdominal and chest discomfort reported by subjects, allowing significantly higher infusion rates of adenosine.(7) Theophylline has also been reported to antagonize the vasorelaxant action of adenosine in human forearm arterioles.(8) In a study in five subjects, theophylline decreased the amounts of adenosine-induced side effects, including chest pain. There was no change in blood pressure or respiratory rate during concurrent adenosine and theophylline.(9) In a study in ten dog and twelve human subjects, the administration of adenosine after hexobendine increased coronary sinus blood flow. Aminophylline administration significantly decreased the coronary vasodilation response to adenosine and hexobendine.(5) In a study in ten healthy subjects, caffeine reduced the mean adenosine-induced increases in systolic blood pressure by 7.2 mmHg and heart rate by 8.4 beats/min when compared to placebo.(2) In another study in ten healthy subjects, caffeine was shown to lower the adenosine-induced response of blood pressure and heart rate.(3) Caffeine has also been reported to reduced adenosine-induced changes in minute ventilation and tidal volume.(3) Aminophylline has been shown to shorten the duration of coronary blood flow response to regadenoson.(3) Coronary flow reserve was 8% lower in patients who received caffeine (200 mg single dose) 2 hours prior to regadenoson administration when compared to subjects who received placebo instead of caffeine.(4)

Xanthine Derivatives/Dipyridamole Injectable

SEVERITY LEVEL: 1-Contraindicated Drug Combination: This drug combination is contraindicated and generally should not be dispensed or administered to the same patient.

MECHANISM OF ACTION: The xanthine derivatives are adenosine receptor antagonists. Concurrent administration may inhibit dipyridamole-induced increases in endogenous plasma adenosine levels, thus decreasing dipyridamole's vasodilator effects.(1)

CLINICAL EFFECTS: Concurrent administration may result in a decrease in dipyridamole's vasodilator effects. This may produce false-negative results during dipyridamole-thallium imaging tests.(1-3)

PREDISPOSING FACTORS: In patients with congestive heart failure and decreased hepatic function, the metabolism of xanthine derivatives may be decreased. These patients may need a longer xanthine-free period prior to dipyridamole-thallium imaging tests.(2)

PATIENT MANAGEMENT: Patients scheduled for dipyridamole-thallium imaging tests should have a xanthine-free period (including caffeine-containing products) for at least 24 hours prior to their exam.(3)

DISCUSSION: In a study in eight male subjects with documented coronary artery disease, intravenous dipyridamole administered during a dipyridamole-thallium 201 SPECT image test produced a significant increase in heart rate, a decrease in blood pressure, and angina in seven patients and ST segment depression in four patients. SPECT imaging showed reversible perfusion defects in myocardial segments supplied by stenotic coronary arteries. When the exam was repeated when the subjects were receiving therapeutic dosages of theophylline, there was no appearance of angina, ST depression, or hemodynamic changes and SPECT imaging shown total absence of reversible perfusion defects.(1) A study in eight patients with coronary artery disease evaluated the effects of caffeine on dipyridamole-201Tl myocardial imaging. The administration of dipyridamole alone resulted in chest pain and ST-segment depression in four patients. Concurrent caffeine infusion decreased the dipyridamole-induced decrease in blood pressure and heart rate. No patients experience chest pain or ST-segment depression. Six patients had false negative test results.(2) Another study found that the attenuation of the hemodynamic response to dipyridamole by caffeine was dose-dependent.(3)

Severe Interaction: Action is required to reduce the risk of severe adverse interaction.

Acetaminophen/Sodium Fusidate

SEVERITY LEVEL: 2-Severe Interaction: Action is required to reduce the risk of severe adverse interaction.

MECHANISM OF ACTION: Sodium fusidate and acetaminophen may inhibit the biotransformation of each other by CYP3A4.(1)

CLINICAL EFFECTS: Concurrent use may result in toxicity from sodium fusidate and acetaminophen.(1)

PREDISPOSING FACTORS: None determined.

PATIENT MANAGEMENT: The Australian manufacturer of sodium fusidate states that concurrent use of sodium fusidate and CYP3A4 biotransformed drugs such as acetaminophen should be avoided.(1)

DISCUSSION: Although the specifics of sodium fusidate metabolic pathways are not known, an interaction between sodium fusidate and drugs that are biotransformed by CYP3A4 is suspected.(1)

Acetaminophen; Nsaids/Alprostadil

SEVERITY LEVEL: 2-Severe Interaction: Action is required to reduce the risk of severe adverse interaction.

MECHANISM OF ACTION: Alprostadil is a prostaglandin E1 product used to maintain patency of a patent ductus arteriosus (PDA).(1) Acetaminophen and nonsteroidal anti-inflammatory (NSAID) agents inhibit prostaglandins and may be used for PDA closure in addition to pain/fever management.(2-4)

CLINICAL EFFECTS: Simultaneous administration of acetaminophen or NSAIDs may result in decreased clinical effects from alprostadil, including reduction in PDA.(1)

PREDISPOSING FACTORS: None determined.

PATIENT MANAGEMENT: Avoid concurrent administration of acetaminophen or NSAIDs in patients on alprostadil for maintaining patency of a patent ductus arteriosus (PDA).(1)

DISCUSSION: NSAIDs and acetaminophen are used as management for patent ductus arteriosus (PDA) closure.(2-4) Alprostadil is used to maintain patency of a PDA.(1) In a case report, a 37-week gestational age neonate with cardiac defects required alprostadil therapy for PDA patency. After multiple doses of acetaminophen for pain, an echocardiogram showed reduction of the PDA requiring increased doses of alprostadil. Additional acetaminophen was discontinued. Follow up echocardiogram showed successful reversal of PDA reduction and alprostadil dose was reduced.(5)

Selected Cyp1a2 Substrates/Viloxazine

SEVERITY LEVEL: 2-Severe Interaction: Action is required to reduce the risk of severe adverse interaction.

MECHANISM OF ACTION: Viloxazine is a strong inhibitor of CYP1A2 and may increase the total exposure of sensitive CYP1A2 substrates.(1) The FDA defines strong inhibition as an increase in drug area-under-curve (AUC) greater than 5-fold.(2)

CLINICAL EFFECTS: Concurrent use of viloxazine with drugs primarily metabolized by CYP1A2 may lead to elevated drug levels and increase the risk of adverse reactions associated with the CYP1A2 substrate.(1)

PREDISPOSING FACTORS: None determined.

PATIENT MANAGEMENT: Drugs linked to this monograph are moderately sensitive to CYP1A2 inhibition. Coadministration of viloxazine with moderately sensitive CYP1A2 substrates is not recommended. If coadministered, dose reduction of the CYP1A2 substrate may be warranted.(1)

DISCUSSION: Concomitant use of viloxazine significantly increases the total exposure, but not peak exposure, of sensitive CYP1A2 substrates, which may increase the risk of adverse reactions associated with these CYP1A2 substrates. In a study, viloxazine increased the AUC of caffeine by almost 6-fold.(1) Though not designed to evaluate drug interactions, the open-label portion of a pediatric randomized controlled trial looking at the association of riluzole concentrations with efficacy and adverse effects found that fluvoxamine (a strong CYP1A2 inhibitor) increased riluzole concentrations by about 2-fold.(3) CYP1A2 substrates linked to this monograph include: caffeine and riluzole.(2,4)

Selected Moderate And Weak Cyp1a2 Inhibitors/Tizanidine

SEVERITY LEVEL: 2-Severe Interaction: Action is required to reduce the risk of severe adverse interaction.

MECHANISM OF ACTION: Moderate and weak CYP1A2 inhibitors may inhibit the metabolism of tizanidine by CYP1A2.(1)

CLINICAL EFFECTS: Concurrent use of moderate and weak CYP1A2 inhibitors may result in elevated levels of and effects from tizanidine, including hypotension, bradycardia, drowsiness, sedation, and decreased psychomotor function.

PREDISPOSING FACTORS: The risk of anticholinergic toxicities including cognitive decline, delirium, falls and fractures is increased in geriatric patients using more than one medicine with anticholinergic properties.(2)

PATIENT MANAGEMENT: The US manufacturer of tizanidine states that concurrent use of tizanidine with inhibitors of CYP1A2 should be avoided.(3) If adverse reactions such as hypotension, bradycardia or excessive drowsiness occur, reduce tizanidine dosage or discontinue tizanidine therapy.(3)

DISCUSSION: In a study, cannabidiol 750 mg twice daily (a weak CYP1A2 inhibitor) increased the maximum concentration (Cmax) and area-under-curve (AUC) of a 200 mg single dose of caffeine (a sensitive CYP1A2 substrate) by 15% and 95%, respectively.(1) In a study in 10 healthy subjects, concurrent fluvoxamine, a strong inhibitor of CYP1A2, increased tizanidine Cmax, AUC, and half-life (T1/2) by 12-fold, 33-fold, and 3-fold, respectively. Significant decreases in blood pressure and increases in drowsiness and psychomotor impairment occurred.(3) In a study in 10 healthy subjects, concurrent ciprofloxacin, a strong inhibitor of CYP1A2, increased tizanidine Cmax and AUC by 7-fold and 10-fold, respectively. Significant decreases in blood pressure and increases in drowsiness and psychomotor impairment occurred.(3) Moderate CYP1A2 inhibitors linked to this monograph include: centanafadine, dipyrone, fexinidazole, genistein, methoxsalen, phenylpropanolamine, pipemidic acid, propranolol, rucaparib, and troleandomycin. Weak CYP1A2 inhibitors linked to this monograph include: allopurinol, artemisinin, belumosudil, caffeine, cannabidiol, curcumin, dan-shen, disulfiram, Echinacea, ginseng, parsley, piperine, ribociclib, simeprevir, thiabendazole, and triclabendazole.(4)

Theophylline Derivatives/Cimetidine

SEVERITY LEVEL: 2-Severe Interaction: Action is required to reduce the risk of severe adverse interaction.

MECHANISM OF ACTION: Cimetidine inhibits the metabolism of theophylline by CYP1A2.(1-10) The duration of cimetidine's inhibitory action is uncertain. Short-term cimetidine therapy appears to reverse rapidly(2) but may persist in prolonged therapy. Increased pentoxifylline serum levels may be the result of an increase in the oral bioavailability of pentoxifylline.(11)

CLINICAL EFFECTS: Concurrent cimetidine and theophylline derivative therapy may result in elevated theophylline derivative concentration levels, prolonged elimination half-life, and decreased clearance.

PREDISPOSING FACTORS: None determined.

PATIENT MANAGEMENT: Theophylline derivative blood levels should be very closely monitored if cimetidine therapy is to be initiated, changed, or discontinued. Theophylline has a narrow therapeutic range; therefore, dosage reductions up to 30-50%(4) should be considered to prevent intoxication when cimetidine therapy is started. Antacids, famotidine, or possibly ranitidine might be more judicious choices than cimetidine in patients receiving theophylline derivatives.

DISCUSSION: It is well documented that cimetidine impairs the elimination of theophylline when the two agents are co-administered to patients.(1-10, 12-22) This interaction has been noted by a variety of routes including continuous intravenous infusion.(22) Reports indicate that with concurrent cimetidine, theophylline plasma concentrations increase, theophylline half-life is prolonged from 29% to 73%(1-3;9,12-14) and theophylline clearance is decreased by 18.5% to 46%.(1-3,9,13,23) Age and smoking do not appear to affect the magnitude of the interaction.(17,18,20) Significant changes can be seen within 24 hours(3,5) and may progress as co-therapy continues.(3) A study involving ten healthy patients demonstrated that concomitant administration of cimetidine significantly decreased the plasma clearance of oxtriphylline.(24) Aminophylline is involved in a similar interaction as theophylline as seen in one case report.(25) In one report cimetidine also decreased the clearance and prolonged the half-life of caffeine.(26,27) A study demonstrated that cimetidine caused a significant increase in plasma levels of pentoxifylline.(11) Information on ranitidine is conflicting. Several studies have shown that ranitidine does not influence theophylline.(9,15,16,19,28,29) One case report noted toxic theophylline levels after ranitidine;(30) however, this case report has been challenged.(31) In another case report, theophylline levels rose from 16.6 mcg/ml to 39.7 mcg/ml(32) when the patient was given ranitidine. Other reports have also noted a reduction in theophylline elimination by ranitidine.(33,34) Famotidine has shown to have no effect on theophylline metabolism in a clinical trial;(35) however, there is one case report of decreased theophylline clearance during famotidine therapy.(36) Dyphylline, a theophylline derivative that is not converted to theophylline in vivo, is not to be expected to interact with cimetidine. A study showed that cimetidine increased the average steady state plasma concentration of pentoxifylline and its metabolite by 25% and 30%, respectively.(37)

Moderate Interaction: Assess the risk to the patient and take action as needed.

Acetaminophen/Busulfan

SEVERITY LEVEL: 3-Moderate Interaction: Assess the risk to the patient and take action as needed.

MECHANISM OF ACTION: Busulfan is eliminated from the body via glutathione conjugation. Acetaminophen reduces glutathione levels in the blood and tissues and therefore could decrease the elimination rate of busulfan.(1,2)

CLINICAL EFFECTS: Concurrent use of acetaminophen may result in elevated levels of, prolonged exposure to, and toxicity from busulfan, including myelosuppression, granulocytopenia, thrombocytopenia, anemia, seizures, hepatic veno-occlusive disease, cardiac tamponade, bronchopulmonary dysplasia, or cellular dysplasia.(1,2)

PREDISPOSING FACTORS: None determined.

PATIENT MANAGEMENT: Use acetaminophen concurrent with busulfan with caution.(1) Consider withholding acetaminophen for 72 hours before and during busulfan therapy. If concurrent use cannot be avoided, monitor patients for busulfan toxicity.

DISCUSSION: Although a small population study in adult patients found no effect of acetaminophen on busulfan clearance,(3) caution is still warranted.(1)

Acetaminophen/Isoniazid

SEVERITY LEVEL: 3-Moderate Interaction: Assess the risk to the patient and take action as needed.

MECHANISM OF ACTION: Isoniazid may induce the metabolism of acetaminophen to its toxic N-acetyl-p-benzoquinone imine (NAPQI) metabolite by CYP2E1.(1)

CLINICAL EFFECTS: Concurrent isoniazid and acetaminophen may result in hepatotoxicity.(1) Symptoms can include nausea, vomiting, jaundice, dark urine, abdominal pain, and unexplained fatigue.

PREDISPOSING FACTORS: The interaction may be more severe in fast acetylators.

PATIENT MANAGEMENT: Concurrent use of acetaminophen in patients treated with isoniazid should be approached with caution. Consider an alternative analgesic agent. If concurrent therapy is warranted, advise patients not to exceed the maximum recommended daily dose of acetaminophen and to immediately report any symptoms of hepatotoxicity.

DISCUSSION: Isoniazid has been shown to induce, after initially inhibiting, the metabolism of acetaminophen to N-acetyl-p-benzoquinone imine (NAPQI), which is hepatotoxicity. Normally, NAPQI is rapidly converted to non-toxic metabolites by glutathione; however, high levels of NAPQI can overwhelm this system.(2-4) In a case report, a patient receiving isoniazid developed severe acetaminophen toxicity following a suicide attempt, despite only having ingested a maximum of 11.5 grams of acetaminophen and having a blood acetaminophen level of 15 mmol/L 13 hours later. Toxicity is usually seen with levels greater than 26 mmol/L.(5) In a retrospective review of 20 deaths in patients taking isoniazid alone or with ethambutol during a 13 year period, two deaths involved patients receiving concurrent isoniazid and acetaminophen.(6,7)

Acetaminophen/Selected Anticoagulants

SEVERITY LEVEL: 3-Moderate Interaction: Assess the risk to the patient and take action as needed.

MECHANISM OF ACTION: Acetaminophen may reduce levels of functional Factor VI, thereby increasing the International Normalized Ratio (INR).(1) In one trial factors II and VII levels were also reduced, thereby increasing the INR. (2)

CLINICAL EFFECTS: Concurrent use of routine acetaminophen, especially at dosages greater than 2 grams/day, and coumarin anticoagulants may result in elevated anticoagulant effects.

PREDISPOSING FACTORS: Routine use of acetaminophen at dosages greater than 2 grams/day may increase the risk of the interaction.

PATIENT MANAGEMENT: Patients receiving routine acetaminophen at dosages greater than 2 grams/day with coumarin anticoagulants should be closely monitored for changes in anticoagulant effects. The dosage of the anticoagulant may need to be adjusted. Patients receiving coumarin anticoagulants should be counseled on the use of acetaminophen.

DISCUSSION: A large systematic review was performed on 72 warfarin drug-drug interactions studies that reported on bleeding, thromboembolic events, or death. Most studies were retrospective cohorts. A meta-analysis of 4 of those studies found a higher rate of clinically significant bleeding in patients on warfarin and non-NSAID analgesics (OR=2.12; 95% CI 1.65-2.73). Increased bleeding risk was also seen in subgroup analyses with acetaminophen (OR=2.32; 95% CI 1.22-4.44).(3) In a study in 11 patients maintained on warfarin, use of acetaminophen (4 grams daily for 14 days) increased INR values by an average of 1.04.(4) In a study in 36 patients maintained on warfarin, the addition of acetaminophen (2 grams/day or 4 grams/day) increased INR values.(5) In a study in 20 patients maintained on warfarin, the addition of acetaminophen (4 grams/day for 14 days) increased average INR values by 1.20 (from 2.6 to 3.45).(6) In a study, 12 patients maintained on various anticoagulants (anisindione, dicoumarol, phenprocoumon, and warfarin) who received 4 weeks of acetaminophen (2.6 grams/day) were compared to 50 subjects maintained on various anticoagulants who did not receive acetaminophen. By the third week of concurrent acetaminophen, prothrombin times increased from 23 seconds to 28.4 seconds. The average warfarin-equivalent dose decreased by 5.8 mg to 4.4 mg. In another phase, 50 subjects maintained on various anticoagulants received acetaminophen (2.6 grams/day for 14 days). The mean prothrombin increase was 3.6 seconds.(7) There have been case reports of increased INRs following concurrent acetaminophen in patients maintained on warfarin(8-11) and acenocoumarol.(12) In contrast to the above reports, other studies have found no effects on acenocoumarol,(14) phenprocoumon,(13-15) or warfarin(16,17) by acetaminophen. In a study in 45 patients maintained on warfarin, the addition of acetaminophen (2 or 3 grams/day for 10 days) increased average INR by 0.7 and 0.67 with 2 grams/day and 3 grams/day, respectively. This increase was apparent by day 3, and a decrease in factor II and VII was observed.(2) A self-controlled case study of 1,622 oral anticoagulant-precipitant drug pairs were reviewed and found 14% of drug pairs were associated with a statistically significant elevated risk of thromboembolism. Concurrent use of warfarin and acetaminophen resulted in a ratio of rate ratios (95% CI) of 1.28 (1.18-1.38).(18) One or more of the drug pairs linked to this monograph have been included in a list of interactions that could be considered for classification as "non-interruptive" in EHR systems. This DDI subset was vetted by an expert panel commissioned by the U.S. Office of the National Coordinator (ONC) for Health Information Technology.

Caffeine-Containing Products/Migalastat

SEVERITY LEVEL: 3-Moderate Interaction: Assess the risk to the patient and take action as needed.

MECHANISM OF ACTION: The mechanism of this interaction is unknown.

CLINICAL EFFECTS: Concurrent use of a caffeine-containing product may result in decreased levels and effectiveness of migalastat.(1)

PREDISPOSING FACTORS: None determined.

PATIENT MANAGEMENT: Avoid coadministration of migalastat with caffeine-containing products. Do not administer caffeine-containing products within 2 hours before and 2 hours after taking migalastat.(1)

DISCUSSION: Coadministration of migalastat with caffeine 190 mg decreased the migalastat maximum concentration (Cmax) by 60% and area-under-curve (AUC) by 55%.(1)

Selected Xanthine Derivatives/Fluvoxamine

SEVERITY LEVEL: 3-Moderate Interaction: Assess the risk to the patient and take action as needed.

MECHANISM OF ACTION: Fluvoxamine may inhibit the metabolism of the xanthine derivatives by CYP1A2.(1,2)

CLINICAL EFFECTS: Concurrent use of fluvoxamine and xanthine derivatives may result in elevated levels of the xanthine derivative and toxicity.

PREDISPOSING FACTORS: None determined.

PATIENT MANAGEMENT: The manufacturer of fluvoxamine recommends that the dose of theophylline be decreased to one-third of the usual daily dose in patients receiving concurrent therapy. Theophylline levels should be closely monitored and patients should be observed for signs of theophylline toxicity.(3) The dosage of theophylline may need to be adjusted if fluvoxamine is discontinued. Patients receiving fluvoxamine should be instructed to consume caffeine containing beverages and/or medications with caution.

DISCUSSION: In a study in 12 healthy subjects, the administration of a single dose of theophylline ethylenediamine (300 mg) on Day 4 of fluvoxamine (50 mg Day 1, 100 mg daily Days 2-6) decreased theophylline total clearance by 70%. The half-life of theophylline increased 2.3-fold (from 6.6 hours to 22 hours).(1) In a study in 12 healthy males, the administration of a single dose of theophylline (375 mg given as 442 mg aminophylline) with fluvoxamine (50 mg twice daily at steady state) decreased theophylline clearance by 3-fold.(3) Fluvoxamine has been shown to inhibit the metabolism of theophylline in vitro.(2) There are four case reports of theophylline toxicity during concurrent fluvoxamine therapy.(4-7) In a study in eight healthy subjects, the administration of a single dose of caffeine (200 mg) on Day 8 of fluvoxamine (50 mg daily Days 1-4, 100 mg daily Days 5-12) decreased caffeine clearance by 80%. The half-life of caffeine increased 5.2-fold (from 5 hours to 31 hours).(8) In a study, seven reports of impaired caffeine clearance were reported in patients whom received single 250mg doses of caffeine together with fluvoxamine (four doses of 100mg over two days). Fluvoxamine reduced the apparent oral clearance of caffeine by 91.3%, and prolonged its elimination half-life by 11.4-fold (from 4.9 hours to 56 hours). There were no changes in the pharmacodynamic effects of caffeine.(9)

Theophylline Derivatives/Lithium

SEVERITY LEVEL: 3-Moderate Interaction: Assess the risk to the patient and take action as needed.

MECHANISM OF ACTION: Theophylline derivatives increase the renal excretion of lithium.

CLINICAL EFFECTS: Decreased levels of lithium which may result in decreased clinical effectiveness.

PREDISPOSING FACTORS: None determined.

PATIENT MANAGEMENT: Lithium levels and response should be monitored in patients in whom theophylline therapy is initiated or withdrawn. Patients receiving concurrent therapy should be monitored for increased adverse effects.

DISCUSSION: In a study involving ten volunteers, the concurrent administration of lithium and theophylline resulted in a significant decrease in lithium serum levels. Upon discontinuation of theophylline, lithium levels and half-life increased, and the clearance of lithium decreased. Individual variability in these parameters was significant. The overall incidence of adverse effects was significantly greater with concurrent therapy including restlessness, tremor, and anorexia. In another study in ten normal subjects, lithium (1200 mg/day for seven days) was administered and it was reported that theophylline infusion (dosed to achieve a plasma level of 14 mcg/ml) increased lithium clearances by 51%. In a case report, reduced lithium levels as well as worsening of manic symptoms occurred after increasing doses of theophylline were administered. It has been shown that aminophylline increases the lithium/creatinine clearance ratio, which may result in decreased serum lithium below the therapeutic level. Caffeine withdrawal has been reported to increase lithium levels in several case reports. This interaction is most important to consider in patients who have been previously sensitive to relapse with decreased lithium levels and in whom levels are maintained at the therapeutic/prophylactic borderline.

*Known hypersensitivity to acetaminophen or any ingredient in the formulation.

*Severe hepatic impairment or severe active liver disease.

• Acetaminophen overdose
• Acute hepatic failure
• Acute hepatitis C

• Chronic kidney disease stage 4 (severe) GFR 15-29 ml/min
• Chronic kidney disease stage 5 (failure) GFr<15 ml/min
• Disease of liver
• Necrotizing enterocolitis
• Peptic ulcer
• Severe hepatic disease

• Cardiac arrhythmia
• Protein-calorie malnutrition
• Seizure disorder
• Shock

The most common adverse effects reported in clinical trials in >=5% of patients receiving IV acetaminophen were nausea, vomiting, headache, and insomnia in adults and nausea, vomiting, constipation, and pruritus in pediatric patients.

Common adverse effects associated with oral or rectal administration of acetaminophen include skin rash, hypersensitivity reactions, nephrotoxicity, and hematologic abnormalities (e.g., anemia, leukopenia, neutropenia, and pancytopenia).

Less Frequent

• Abnormal hepatic function tests

Rare or Very Rare

• Acute generalized exanthematous pustulosis
• Acute hepatic failure
• Agranulocytosis
• Allergic dermatitis
• Angioedema
• Drug-induced hepatitis
• Extrasystoles
• Laryngeal edema
• Leukopenia
• Maculopapular rash
• Neutropenic disorder
• Stevens-johnson syndrome
• Thrombocytopenic disorder
• Toxic epidermal necrolysis

More Frequent

• Gastrointestinal irritation
• Insomnia
• Nervousness

Less Frequent

• Excitement

Rare or Very Rare

• Agitation
• Erythema
• Hyperesthesia
• Hyperglycemia
• Irritability
• Medication overuse headache
• Nausea
• Scotomata
• Skin rash
• Symptoms of anxiety
• Tachycardia
• Tinnitus
• Tremor
• Urticaria
• Vomiting

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Use of IV acetaminophen for analgesia in pediatric patients >=2 years of age is supported by evidence from adequate and well-controlled studies of IV acetaminophen in adults and additional safety and pharmacokinetic data from 483 pediatric patients ranging in age from neonates to adolescents. Efficacy of IV acetaminophen for analgesia has not been established in pediatric patients <2 years of age.

Use of IV acetaminophen for fever in pediatric patients <2 years of age, including premature neonates (postmenstrual age of at least 32 weeks) is supported by evidence from adequate and well-controlled studies of IV acetaminophen in adults, clinical data in pediatric patients >=2 years of age, and additional safety and pharmacokinetic data from 239 patients <2 years of age, including neonates with a postmenstrual age of at least 32 weeks.

Epidemiologic data regarding oral acetaminophen use in pregnant females have shown no increased risk of major congenital malformations in infants exposed in utero to the medication. In a large population-based prospective cohort study involving more than 26,000 females with live-born singleton infants who were exposed to oral acetaminophen during the first trimester of pregnancy, no increase in the risk of congenital malformations was observed in exposed children compared with a control group of unexposed children; rate of congenital malformations (4.3%) was similar to the rate in the general population. A population-based, case-control study from the National Birth Defects Prevention Study also found no increase in the risk of major birth defects in a group of 11,610 children who had been exposed to acetaminophen during the first trimester of pregnancy compared with a control group of 4,500 children.

Animal reproduction studies in pregnant rats given oral acetaminophen during organogenesis at dosages up to 0.85 times the maximum recommended human daily dosage (4 g daily, based on body surface area comparison) showed evidence of fetotoxicity (reduced fetal weight and length) and a dose-related increase in bone variations (reduced ossification and rudimentary rib changes); the offspring showed no evidence of external, visceral, or skeletal malformations. When pregnant rats received oral acetaminophen throughout gestation at a dosage of 1.2 times the maximum recommended human daily dosage, areas of necrosis occurred in both the liver and kidney of pregnant rats and fetuses; these effects did not occur in animals given acetaminophen at dosages of 0.3 times the maximum recommended human dosage.

In a continuous breeding study in which pregnant mice were given acetaminophen at dosages approximately equivalent to 0.43, 0.87, or 1.7 times the maximum recommended human daily dosage (based on body surface area comparison), a dose-related reduction in body weight of the fourth and fifth litter offspring of the treated mating pair occurred during lactation and following weaning at all dosages studied. Animals receiving the highest dosage had a reduced number of litters per mating pair, male offspring with an increased percentage of abnormal sperm, and reduced birth weights in the next-generation pups.

The manufacturer states that there are no animal reproductive studies with IV acetaminophen.

A correlation between acetaminophen use during pregnancy and neurodevelopmental disorders in children, including autism and attention-deficit/hyperactivity disorder (ADHD), has been observed in studies; however, inconsistent findings have been reported in the literature and a causal relationship has not been established. Results of 2 high-quality studies with sibling controls indicate prenatal exposure to acetaminophen was not associated with an increased risk of a neurodevelopmental diagnosis. There are unmeasured familial confounding factors that may explain the apparent association observed in some studies. Acetaminophen is considered the preferred analgesic/antipyretic for use during pregnancy because of its relatively favorable safety profile compared with other common analgesics. The American College of Obstetricians and Gynecologists (ACOG) supports use of acetaminophen during pregnancy when taken as needed in moderation and after consultation with a doctor. There is no evidence to support any risk associated with appropriate use of acetaminophen during pregnancy. However, untreated maternal fever and pain pose risks such as neural tube defects and preterm birth, necessitating a balanced approach.

There are no adequate and well-controlled studies in pregnant women. In studies performed in adult animals, caffeine (as caffeine base) administered to pregnant mice as sustained-release pellets at 50 mg/kg (less than the maximum IV loading dose for neonates on a mg/m2 basis) during the period of organogenesis caused a low incidence of cleft palate and exencephaly in fetuses. Based on data from a large retrospective epidemiologic study and from a large retrospective case-control study in humans, it appears that use of caffeine during pregnancy has little, if any, effect on the outcome of pregnancy. Although caffeine use during pregnancy does not appear to be associated with substantial risk, most clinicians recommend that pregnant women avoid or limit their consumption of foods, beverages, and drugs containing caffeine, since caffeine crosses the placenta.

It is not known if IV acetaminophen is excreted into human milk; however, data suggest following oral administration of acetaminophen, infant exposure is approximately 1-2% of the weight-adjusted maternal daily dosage. Limited published studies report acetaminophen passes rapidly into human milk with similar levels noted in milk and plasma. A case of rash in a breast-fed infant has been reported; the rash resolved when the mother discontinued acetaminophen use and recurred when she resumed acetaminophen therapy. The developmental and health benefits of breast-feeding should be considered along with the mother's clinical need for acetaminophen and any potential adverse effects on the breast-fed infant from exposure to the medication or to the underlying maternal condition.

Caffeine is distributed into the milk of nursing women. Milk-to-plasma ratios of 0.5-0.76 have been reported. The amount of caffeine ingested from usual quantities of caffeinated beverages is considered compatible with breast-feeding; however, caffeine may accumulate in nursing infants following moderate to heavy maternal consumption of caffeine, resulting in irritability and poor sleeping patterns.

The manufacturer makes no specific dosage recommendations for geriatric patients. Although no overall differences in efficacy or safety were observed between geriatric and younger patients, and other clinical experience revealed no evidence of age-related differences, the possibility that some older patients may exhibit increased sensitivity to IV acetaminophen cannot be ruled out.

When the total number of patients studied in clinical trials of IV acetaminophen is considered, 15% were >=65 years of age, while 5% were >=75 years of age. Although no overall differences in efficacy or safety were observed between geriatric and younger patients, and other clinical experience revealed no evidence of age-related differences, the possibility some older patients may exhibit increased sensitivity to acetaminophen cannot be ruled out.