Metronidazole and Phenobarbital Interaction
Drug interaction information between Metronidazole and Phenobarbital.
Metronidazole and Phenobarbital have a documented minor interaction in FDA labeling.
FDA drug labeling documents a minor-severity interaction between Metronidazole and Phenobarbital. 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
Phenobarbital makes your liver break down metronidazole faster than normal. This can lower the amount of medicine in your blood and make it less effective at fighting infections.
FDA Label Information
Drugs that Induce CYP450 Enzymes The simultaneous administration of drugs that induce microsomal liver enzymes, such as phenytoin or phenobarbital, may accelerate the elimination of metronidazole, resulting in reduced plasma levels; impaired clearance of phenytoin has also been reported.
Metronidazole Also Interacts With
- Theophylline major
- Lithium minor
- Disulfiram minor
- Cimetidine minor
- Phenytoin minor
Phenobarbital Also Interacts With
- Nirmatrelvir/Ritonavir major
- Ranolazine major
- Voriconazole major
- Canagliflozin moderate
- Cenobamate moderate
Frequently Asked Questions
What SPL severity band is on file for Metronidazole + Phenobarbital?
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?
Phenobarbital has 59 labeled pairs on file vs Metronidazole's 10. In the minor band specifically: Metronidazole has 9 (also Lithium, Disulfiram, Cimetidine, Phenytoin); Phenobarbital has 43 (also Estradiol, Doxycycline, Progesterone, Griseofulvin).
What mechanism field is on file for Metronidazole and Phenobarbital?
Phenobarbital makes your liver break down metronidazole faster than normal. This can lower the amount of medicine in your blood and make it less effective at fighting infections.
Phenobarbital carries more major-labeled pairs than Metronidazole
SPL source-record counts only. This page's band is minor · classes on file: Metronidazole (Nitroimidazole Antibiotic); Phenobarbital (Anticonvulsant (Barbiturate)).
Phenobarbital has 3 major-labeled pairs vs Metronidazole's 1. This page itself is in the minor band, not the major band.
1 major · 0 moderate · 9 minor · 10 total
- Other minor peers for Metronidazole
- Lithium
- Disulfiram
- Cimetidine
- Phenytoin
3 major · 13 moderate · 43 minor · 59 total
- Other minor peers for Phenobarbital
- Estradiol
- Doxycycline
- Progesterone
- Griseofulvin
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.