Colesevelam and Phenytoin: Interaction Details
AI-assisted, pharmacist-reviewed · Source data updated Aug 8, 2026 · Sources: FDA labeling, DDInter 2.0, cited literature
Phenytoin
Colesevelam
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 phenytoin exposure
Interaction Deep Dive
Concurrent use of phenytoin and colesevelam may result in decreased exposure to phenytoin. Postmarketing cases of increased seizure activity have been reported among patients receiving phenytoin and colesevelam concomitantly. In patients requiring coadministration of these agents, phenytoin should be taken at least 4 hours prior to colesevelam. Alternatively, patients should be monitored for an adequate response to phenytoin therapy and the phenytoin dose should be adjusted appropriately1.
Why it happens (mechanism)
Reduced absorption of phenytoin by nonspecific binding with colesevelam
Literature reports
1 report — tap to read
a) Increased seizure activity and reduced phenytoin levels were reported among patients receiving phenytoin and colesevelam concomitantly during postmarketing use. Frequency and causality of these events have not been established 1.
Common questions
Can I take Colesevelam and Phenytoin together?
Decreased phenytoin exposure Always confirm with your pharmacist or prescriber before making any change.
How serious is the Colesevelam and Phenytoin interaction?
It is rated moderate. Can be significant — usually manageable with monitoring.
How quickly could this interaction happen?
The documented onset is "rapid". Effects can appear quickly, often within about 24 hours of combining the drugs.
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 Colesevelam 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 (1)
- Product Information: WELCHOL oral tablets, colesevelam hcl oral tablets. Daiichi Sankyo Inc, Parsippany, NJ, 2008. DailyMed
Keep reading about Phenytoin
Keep reading about Colesevelam
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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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