Acetaminophen And Codeine and Phenytoin: Interaction Details
AI-assisted, pharmacist-reviewed · AI content regenerated Jul 11, 2026 · Source data updated Jul 1, 2026 · Sources: FDA labeling, DDInter 2.0, cited literature
Acetaminophen And Codeine
Phenytoin
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.
Your pain medicine contains acetaminophen (Tylenol) plus codeine, and you also take phenytoin (Dilantin) for seizures. Phenytoin speeds up the liver's breakdown of acetaminophen. That can do two things: your pain relief may not last as long, and your body makes more of a harmful byproduct that can be hard on the liver, especially with large or long-term doses.
This does not mean you have to stop either medicine. It mostly matters when acetaminophen is taken in big amounts or every day for a long time. Please talk with your pharmacist or doctor about how much acetaminophen is safe for you. They can adjust things and keep an eye on your liver so you stay protected.
Effect: Phenytoin induces CYP3A4 (and related pathways), increasing acetaminophen metabolism by >40% and reducing its half-life ~25%. This shifts more drug toward hepatotoxic metabolites (NAPQI) while shortening analgesic duration. The codeine component is not the concern here.
- Direction: reduced acetaminophen efficacy plus increased hepatotoxicity risk.
- Onset: delayed. Evidence: probable. Severity: moderate.
- Management: Avoid large and/or chronic acetaminophen dosing during phenytoin therapy. Individualize dose, favor lowest effective amount, and monitor LFTs/clinical signs of hepatotoxicity, particularly with sustained use or overdose scenarios.
What happens
Decreased acetaminophen effectiveness and an increased risk of hepatotoxicity
Interaction Deep Dive
Studies show that phenytoin raises acetaminophen metabolism by more than 40% while shortening the acetaminophen half-life by roughly 25%. Since this leads to the formation of toxic acetaminophen metabolites, individuals using phenytoin at the same time may face an elevated risk of acetaminophen toxicity. The period of analgesia may also be reduced3.
Why it happens (mechanism)
Induction of CYP3A4-mediated acetaminophen hepatic metabolism
How to manage this interaction
Keep taking both medicines as prescribed. The concern is mainly with large or long-term acetaminophen use, not an occasional dose.
- Ask your pharmacist or doctor what daily acetaminophen limit is right for you while on phenytoin.
- Avoid regularly taking high doses over many days without guidance.
- Your care team may check your liver and monitor you more closely if you need acetaminophen often.
- Watch for signs of liver trouble: nausea, belly pain on the upper right side, unusual tiredness, yellowing of the skin or eyes, or dark urine, and report them.
Your team can tailor the dose and monitoring to keep you safe.
Management is individual — confirm any change with your pharmacist or prescriber.
Literature reports
2 reports — tap to read
a) Acetaminophen undergoes hepatic metabolism through two distinct pathways in the liver. Roughly 80% to 90% of an acetaminophen dose is conjugated by a phase II reaction to either glucuronic acid or sulfate, neither of which is toxic. The rest is processed by a phase I cytochrome P450 (CYP) reaction into a reactive, electrophilic intermediate called N-acetyl-p-benzoquinone imine (NAPQI). NAPQI is quickly conjugated with glutathione, yielding mercapturic acid and other nontoxic products that are excreted in the urine. When glutathione reserves are exhausted or NAPQI generation rises, the detoxification of NAPQI declines and NAPQI builds up, attaches to hepatic tissue macromolecules, and produces oxidative stress, tissue injury, or necrosis. Induction of the CYP system raises the rate of drug metabolism and likewise raises the rate at which NAPQI is produced. Consequently, the quantity of NAPQI generated by a therapeutic dose of acetaminophen in a patient taking a CYP-inducing agent may equal the quantity of NAPQI generated by a toxic dose of acetaminophen in someone who is not induced 4.
b) A 55-year-old woman who smoked and had chronic obstructive pulmonary disease acquired community-acquired pneumonia and was started on cefprozil 500 mg twice daily. Her other medications comprised 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. When she was admitted to the hospital for placement of an inferior vena cava (IVC) filter, her hepatic transaminases, lactose dehydrogenase, and alkaline phosphatase were markedly elevated. A review of her drugs revealed that three separate analgesics, each containing acetaminophen, had been prescribed and taken by the patient. Her daily acetaminophen intake ranged from 1300 mg to 6200 mg daily. Since phenytoin was also being administered, acetaminophen-induced liver injury resulting from enzyme induction was suspected. All acetaminophen-containing products were stopped, and the patient's hepatic values were normal two and six weeks afterward 4.
Common questions
Can I take Acetaminophen And Codeine and Phenytoin together?
Phenytoin makes acetaminophen wear off faster and may raise liver-toxicity risk, so avoid large or long-term acetaminophen doses and ask your care team how much is safe for you. Always confirm with your pharmacist or prescriber before making any change.
How serious is the Acetaminophen And Codeine 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 is the Acetaminophen And Codeine and Phenytoin interaction managed?
Keep taking both medicines as prescribed. The concern is mainly with large or long-term acetaminophen use, not an occasional dose. Ask your pharmacist or doctor what daily acetaminophen limit is right for you while on phenytoin. Avoid regularly taking high doses over many days without guidance. Your care team may check your liver and monitor you more closely if you need acetaminophen often. Watch… Management is individual — always follow your own care team's guidance.
How strong is the evidence for this interaction?
The evidence is graded "probable". Good supporting evidence, though not definitively proven.
Questions for your pharmacist
- Does my dose of Acetaminophen And Codeine 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 (4)
- Product Information: Codeine sulfate oral solution, codeine sulfate oral solution. Hikma Pharmaceuticals USA Inc(per FDA), Berkeley Heights, NJ, 2023. DailyMed
- Product Information: Codeine sulfate oral tablets, codeine sulfate oral tablets. Hikma Pharmaceuticals USA Inc. (per FDA), Berkeley Heights, NJ, 2023. DailyMed
- 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. PubMed
- 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 Acetaminophen And Codeine
Keep reading about Phenytoin
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