Doxycycline and Phenytoin Interaction
Drug interaction information between Doxycycline and Phenytoin.
Doxycycline and Phenytoin have a documented minor interaction in FDA labeling.
FDA drug labeling documents a minor-severity interaction between Doxycycline and Phenytoin. Major interactions are generally avoided, moderate ones may need monitoring or a dose adjustment, and minor ones are usually low-risk. This page shows the documented mechanism and guidance. Label-documented interactions are not a complete safety review, so always confirm your own medications with a pharmacist or doctor. Educational information, not medical advice.
How They Interact
Phenytoin speeds up how quickly your body breaks down and removes doxycycline from your system.
What To Do
Your healthcare provider may need to change your dose of doxycycline to ensure it stays effective.
FDA Label Information
Barbiturates, carbamazepine, and phenytoin decrease the half-life of doxycycline.
Doxycycline Also Interacts With
- Tetracycline moderate
- Rifampin moderate
- Carbamazepine minor
- Bismuth Subsalicylate minor
- Doxycycline Hyclate minor
Phenytoin Also Interacts With
- Nirmatrelvir/Ritonavir major
- Posaconazole major
- Ranolazine major
- Voriconazole major
- Apixaban moderate
Frequently Asked Questions
Can I take Doxycycline and Phenytoin together?
This is a minor interaction. Your healthcare provider may need to change your dose of doxycycline to ensure it stays effective.
How serious is the interaction between Doxycycline and Phenytoin?
This interaction is classified as "minor" severity by the FDA. Minor interactions are unlikely to cause significant problems but should still be mentioned to your healthcare provider.
Why do Doxycycline and Phenytoin interact?
Phenytoin speeds up how quickly your body breaks down and removes doxycycline from your system.
Understanding the Doxycycline and Phenytoin Interaction
FDA-approved prescribing information for these two drugs flags their combination as a minor-severity interaction. Doxycycline belongs to the Tetracycline Antibiotic class and Phenytoin belongs to the Anticonvulsant (Hydantoin) class - two categories that can collide when co-prescribed. The mechanism described in FDA labeling is: Phenytoin speeds up how quickly your body breaks down and removes doxycycline from your system. Severity tiers matter: major flags generally advise avoidance, moderate flags often require monitoring or dose adjustment, and minor flags may only call for awareness.
Context around a specific patient determines real-world impact. Doxycycline has 9 total documented interactions on file in this dataset, and Phenytoin has 147. Each additional medication compounds the interaction surface, which is why pharmacists run full-profile checks rather than evaluating one pair at a time. FDA-derived guidance for this pair is: Your healthcare provider may need to change your dose of doxycycline to ensure it stays effective. Timing of doses, renal and hepatic function, age, and other concurrent prescriptions all shape whether a labeled interaction matters clinically.
An interaction flag is not a verdict. A large share of labeled interactions are managed routinely in clinical practice, the fix may be as simple as spacing doses or adding a monitoring test. Others require the prescriber to choose a different medication entirely. This page surfaces FDA-sourced labeling and openFDA data for educational purposes only; it is not medical advice and cannot account for your full clinical picture. Never start, stop, or adjust either Doxycycline or Phenytoin based on a web page, speak with your prescriber or pharmacist before making any change.
Sources: FDA Drug Labels (SPL) via openFDA (2026). This is for informational purposes only and is not medical advice. Always consult your healthcare provider about drug interactions.
Read our methodology - how this data is sourced, computed, and verified.