What the FDA labels say about taking them together
Metronidazole and phenobarbital
Metronidazole's FDA label of September 3, 2024 names phenobarbital in its drug interactions section; the phenobarbital labels PlainMeds reads (2) do not name metronidazole. The metronidazole label: “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; …”
Check it at the source: the metronidazole FDA label on DailyMed (effective September 3, 2024, Aurobindo Pharma Limited); the phenobarbital FDA label on DailyMed (effective August 27, 2026).
What does the metronidazole label say about phenobarbital?
Metronidazole label, drug interactionsDrugs 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 label (Baxter Healthcare Corporation, June 12, 2025), drug interactionsDrugs that Induce CYP450 Enzymes The simultaneous administration of drugs that induce microsomal liver enzyme activity, such as phenytoin or phenobarbital, may accelerate the elimination of metronidazole and therefore decrease its efficacy.
Source: FDA drug label (Metronidazole, NDA018900), B. Braun Medical Inc., effective April 23, 2026 (SPL set fd191c06-9d3b-4e46-9f79-90f6af614947), via openFDA
Source: FDA drug label (Metronidazole, NDA018657), Baxter Healthcare Corporation, effective June 12, 2025 (SPL set d2112f37-5080-4a19-bcc3-6386b49abe1d), via openFDA
Source: FDA drug label (Metronidazole), Aurobindo Pharma Limited, effective September 3, 2024 (SPL set f68397ee-67db-472e-8607-4300064e37df), via openFDA; the full label on DailyMed
What does the phenobarbital label say about metronidazole?
Phenobarbital's label (effective August 27, 2026) and its 1 other brand-application label do not name metronidazole in these sections. A label lists what its studies and reports found; it is not a complete list.
Source: FDA drug label (Phenobarbital), Chartwell RX, LLC, effective August 27, 2026 (SPL set 74428144-fb24-40a5-a670-3b876e2d116c), via openFDA; the full label on DailyMed
These are the labels' own sentences. PlainMeds does not grade interactions; a pharmacist or prescriber can say what they mean for one person's medicines.
Both medicines on the record
| Metronidazole | Phenobarbital | |
|---|---|---|
| Labeled for | Bacterial infections | Seizures and epilepsy |
| Boxed warning | Yes | No |
| Pharmacy cost (NADAC) | $0.102 per tablet 250 mg tablet | $0.213 per tablet 100 mg tablet |
| Shortage now | In shortage | A presentation being discontinued |
Source: FDA NDC Directory, CMS NADAC of October 7, 2026 and the FDA Drug Shortage Database (FDA files October 8, 2026)
What next?
- Check more medicines together
- up to five at once