Acetaminophen and Phenytoin: Interaction Details
AI-assisted, pharmacist-reviewed · Source data updated Jul 10, 2026 · Sources: FDA labeling, DDInter 2.0, cited literature
Phenytoin
Acetaminophen
How we grade severity & evidence
Severity levels
- Contraindicated: These should generally not be used together.
- Major: Potentially serious — often needs a change or close monitoring.
- Moderate: Can be significant — usually manageable with monitoring.
- Minor: Usually limited clinical impact.
Evidence grades
- Established: Well documented — supported by controlled studies or strong clinical data.
- Probable: Good supporting evidence, though not definitively proven.
- Suspected: Some evidence suggests this interaction, but it is not well established.
- Possible: Limited or conflicting evidence; the interaction may occur.
- Theoretical: Predicted from the drugs' pharmacology; not yet confirmed in people.
Ratings come from the documented interaction literature and are reviewed by a pharmacist. They describe the documented risk of the combination, not what will necessarily happen to you — your dose, timing, and health picture all matter.
What happens
Decreased acetaminophen effectiveness and an increased risk of hepatotoxicity
Interaction Deep Dive
Phenytoin has been demonstrated to increase the metabolism of acetaminophen by greater than 40% and to decrease the acetaminophen half-life by about 25%. Because of the production of toxic acetaminophen metabolites, patients may be at a greater risk for acetaminophen toxicity when taking phenytoin concurrently. In addition, the duration of analgesia may be shorter1.
Why it happens (mechanism)
Induction of CYP3A4-mediated acetaminophen hepatic metabolism
Literature reports
2 reports — tap to read
a) Acetaminophen is hepatically metabolized in the liver via two separate pathways. Approximately 80% to 90% of an acetaminophen dose is conjugated by a phase II reaction to either glucuronic acid or sulfate, both of which are nontoxic. The remainder is metabolized by a phase I cytochrome P450 (CYP) reaction to a reactive, electrophilic intermediate, N-acetyl-p-benzoquinone imine (NAPQI). NAPQI is rapidly conjugated with glutathione to form mercapturic acid and other nontoxic products which are eliminated in the urine. If glutathione stores are depleted or NAPQI production is increased, detoxification of NAPQI is reduced and NAPQI accumulates, binds to hepatic tissue macromolecules, and causes oxidative stress, tissue damage, or necrosis. Induction of the CYP system increases the rate of drug metabolism and also increases the rate of production of NAPQI. Therefore, the amount of NAPQI produced by a therapeutic dose of acetaminophen in a patient receiving a CYP-inducing drug may be the same as the amount of NAPQI produced by a toxic dose of acetaminophen in a non-induced person 2.
b) A 55-year-old female smoker with chronic obstructive pulmonary disease developed community-acquired pneumonia and was placed on cefprozil 500 mg twice daily. Other medications included phenytoin 400 mg alternating every other day with 300 mg, warfarin 6 mg daily, diltiazem 30 mg four times daily, cisapride 20 mg four times daily, famotidine 40 mg daily, and paroxetine 20 mg daily. On admission to the hospital for placement of an inferior vena cava (IVC) filter, hepatic transaminases, lactose dehydrogenase, and alkaline phosphatase were significantly elevated. Drug review showed that three different analgesics, all containing acetaminophen, were prescribed and used by the patient. Daily doses of acetaminophen ranged from 1300 mg to 6200 mg daily. Because phenytoin was also being given, acetaminophen-induced liver injury secondary to enzyme induction was suspected. All acetaminophen products were discontinued, and the patient's hepatic values were normal two and six weeks later 2.
Common questions
Can I take Acetaminophen and Phenytoin together?
Decreased acetaminophen effectiveness and an increased risk of hepatotoxicity Always confirm with your pharmacist or prescriber before making any change.
How serious is the Acetaminophen and Phenytoin interaction?
It is rated moderate. Can be significant — usually manageable with monitoring.
How quickly could this interaction happen?
The documented onset is "delayed". Effects tend to build up gradually over days to weeks.
How strong is the evidence for this interaction?
The evidence is graded "probable". Good supporting evidence, though not definitively proven.
From our Q&A
Real reader questions about these medications, each personally answered by our pharmacist:
Questions for your pharmacist
- Does my dose of Acetaminophen or Phenytoin need adjusting while I take them together?
- What symptoms should prompt me to call you or my prescriber right away?
- Does the timing of my doses matter for this combination?
- Is there anything you'd monitor while I'm on both?
References (2)
- Miners JO, Attwood J, & Birkett DJ: Determinants of acetaminophen metabolism: effect of inducers and inhibitors of drug metabolism on acetaminophen's metabolic pathways. Clin Pharmacol Ther 1984; 35:480-486. DOI
- Brackett CC & Bloch JD: Phenytoin as a possible cause of acetaminophen hepatotoxicity: case report and review of the literature. Pharmacotherapy 2000; 20:229-233. PubMed
Keep reading about Phenytoin
Keep reading about Acetaminophen
These medications also interact with supplements
Prescription drugs aren't the whole picture — herbal and dietary supplements can interact with them too. From the evidence-graded Natural Medicines database:
major · moderate · minor — check everything you take with our drug–supplement interaction checker.
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