Metformin and Phenytoin Interaction
Drug interaction information between Metformin and Phenytoin.
Metformin and Phenytoin have a documented minor interaction in FDA labeling.
FDA drug labeling documents a minor-severity interaction between Metformin and Phenytoin. This page displays the available source-record category and label text. The category is not an individual risk estimate or medication-combination direction.
How They Interact
Phenytoin can interfere with blood sugar control, which may make metformin less effective at lowering your blood sugar.
FDA Label Information
Examples: Thiazides and other diuretics, corticosteroids, phenothiazines, thyroid products, estrogens, oral contraceptives, phenytoin, nicotinic acid, sympathomimetics, calcium channel blockers, and isoniazid.
Metformin Also Interacts With
- Bictegravir/Emtricitabine/Tenofovir moderate
- Dolutegravir/Lamivudine moderate
- Empagliflozin moderate
- Empagliflozin/Linagliptin moderate
- Ertugliflozin moderate
Phenytoin Also Interacts With
- Nirmatrelvir/Ritonavir major
- Posaconazole major
- Ranolazine major
- Voriconazole major
- Apixaban moderate
Frequently Asked Questions
What SPL severity band is on file for Metformin + Phenytoin?
Compiled FDA labeling labels this pair minor. That is a source-record field, not an individual medication-combination decision.
Which drug in this pair has the larger labeled-interaction portfolio?
Phenytoin has 147 labeled pairs on file vs Metformin's 27. In the minor band specifically: Metformin has 17 (also Topiramate, Dolutegravir, Ranolazine, Cimetidine); Phenytoin has 120 (also Atorvastatin, Omeprazole, Simvastatin, Methylphenidate).
What mechanism field is on file for Metformin and Phenytoin?
Phenytoin can interfere with blood sugar control, which may make metformin less effective at lowering your blood sugar.
Phenytoin carries more major-labeled pairs than Metformin
SPL source-record counts only. This page's band is minor · classes on file: Metformin (Biguanide); Phenytoin (Anticonvulsant (Hydantoin)).
Phenytoin has 4 major-labeled pairs vs Metformin's 0. This page itself is in the minor band, not the major band.
0 major · 10 moderate · 17 minor · 27 total
- Other minor peers for Metformin
- Topiramate
- Dolutegravir
- Ranolazine
- Cimetidine
4 major · 23 moderate · 120 minor · 147 total
- Other minor peers for Phenytoin
- Atorvastatin
- Omeprazole
- Simvastatin
- Methylphenidate
Sources: FDA Drug Labels (SPL) via openFDA (2026). This is for informational purposes only and is not medical advice. This registry does not provide individual medication-combination directions.
Read our methodology - how this data is sourced, computed, and verified.