Phenytoin and Amitriptyline: Interaction Details
AI-assisted, pharmacist-reviewed · AI content regenerated Jul 11, 2026 · Source data updated Jul 2, 2026 · Sources: FDA labeling, DDInter 2.0, cited literature
Amitriptyline
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.
These two medicines can affect each other in two ways. Amitriptyline (Elavil) may slow down how your body clears phenytoin (Dilantin), so phenytoin levels can climb higher. That matters because phenytoin has a narrow safety window, and too much can cause dizziness, unsteadiness, slurred speech, or double vision.
At the same time, phenytoin can speed up how your body breaks down amitriptyline, which may make the antidepressant work less well. The good news is this is very manageable. Your care team can check phenytoin levels and adjust doses to keep both medicines working safely. Keep taking both as prescribed and let your pharmacist or doctor know how you feel.
Bidirectional interaction, theoretical evidence, rated major.
- Phenytoin exposure: TCAs may inhibit phenytoin metabolism, increasing phenytoin levels. Given phenytoin's narrow therapeutic index and nonlinear kinetics, small changes can produce toxicity (nystagmus, ataxia, sedation).
- TCA exposure: Phenytoin is a hepatic enzyme inducer (CYP), so it can enhance amitriptyline metabolism, reducing TCA levels and potentially antidepressant efficacy.
- Onset: unspecified; induction effects typically develop over days to weeks.
Management: Monitor phenytoin serum concentrations and clinical signs of toxicity. Assess antidepressant response; amitriptyline dose may need individualized adjustment. Titrate under care-team supervision.
What happens
An increased risk of phenytoin exposure and reduced TCA exposure
Interaction Deep Dive
When phenytoin is taken together with tricyclic antidepressants (TCA), phenytoin exposure may rise. Since phenytoin acts as an enzyme inducer, it can accelerate antidepressant metabolism, leading to lower TCA exposure12.
Why it happens (mechanism)
Inhibition of phenytoin metabolism
How to manage this interaction
This interaction is manageable with monitoring and dose tailoring. Here is what usually happens:
- Your team may check phenytoin blood levels and watch for signs it is running too high (dizziness, unsteadiness, double vision, drowsiness).
- They may watch whether your amitriptyline is still working well, since phenytoin can lower its levels.
- Either dose may need to be adjusted and individualized by your care team.
What to do: Keep taking both exactly as prescribed. Do not stop or change either one on your own. Tell your pharmacist or prescriber if you notice new dizziness, balance problems, or if your mood symptoms return.
Management is individual — confirm any change with your pharmacist or prescriber.
Literature reports
1 report — tap to read
a) Several case reports have suggested that imipramine suppresses the metabolism of phenytoin, leading to higher serum phenytoin concentrations. Tricyclic antidepressants (TCAs) can reduce the seizure threshold in epileptic patients who are stabilized on anticonvulsants. In theory, since phenytoin acts as an enzyme inducer, the metabolism of antidepressants could be enhanced, resulting in decreased TCA serum levels 12.
Common questions
Can I take Phenytoin and Amitriptyline together?
Taking amitriptyline with phenytoin can raise phenytoin levels (toward toxicity) while lowering amitriptyline's effect, so your care team may monitor phenytoin levels and adjust doses. Keep taking both as prescribed and report any dizziness, unsteadiness, or return of depression symptoms. Always confirm with your pharmacist or prescriber before making any change.
How serious is the Phenytoin and Amitriptyline interaction?
It is rated major. Potentially serious — often needs a change or close monitoring.
How quickly could this interaction happen?
The documented onset is "unspecified". The timing of this interaction is not well characterized.
How is the Phenytoin and Amitriptyline interaction managed?
This interaction is manageable with monitoring and dose tailoring. Here is what usually happens: Your team may check phenytoin blood levels and watch for signs it is running too high (dizziness, unsteadiness, double vision, drowsiness). They may watch whether your amitriptyline is still working well, since phenytoin can lower its levels. Either dose may need to be adjusted and individualized by yo… Management is individual — always follow your own care team's guidance.
How strong is the evidence for this interaction?
The evidence is graded "theoretical". Predicted from the drugs' pharmacology; not yet confirmed in people.
From our Q&A
Real reader questions about these medications, each personally answered by our pharmacist:
Questions for your pharmacist
- Does my dose of Phenytoin or Amitriptyline 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 a safer alternative to one of these medications for me?
References (2)
Keep reading about Amitriptyline
Keep reading about Phenytoin
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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