What the FDA labels say about taking them together
Nelfinavir and phenobarbital
Nelfinavir's FDA label of December 11, 2025 names phenobarbital in its drug interactions section; the phenobarbital labels PlainMeds reads (2) do not name nelfinavir. The nelfinavir label: “Concentrations of nelfinavir may be decreased. VIRACEPT may not be effective due to decreased nelfinavir plasma concentrations in patients taking these agents concomitantly. …”
Check it at the source: the nelfinavir FDA label on DailyMed (effective December 11, 2025, AGOURON); the phenobarbital FDA label on DailyMed (effective August 27, 2026).
What does the nelfinavir label say about phenobarbital?
Nelfinavir label, drug interactionsConcomitant Drug Class/Drug Name: Carbamazepine Phenobarbital Phenytoin; Effect on Concentration: ↓ nelfinavir ↓ phenytoin; Clinical Comment: Concentrations of nelfinavir may be decreased. VIRACEPT may not be effective due to decreased nelfinavir plasma concentrations in patients taking these agents concomitantly. Phenytoin plasma/serum concentrations should be monitored; phenytoin dose may require adjustment to compensate for altered phenytoin concentration.
Source: FDA drug label (VIRACEPT), AGOURON, effective December 11, 2025 (SPL set e72c2bc6-9462-4a2e-8e1d-b97592376cbd), via openFDA; the full label on DailyMed
What does the phenobarbital label say about nelfinavir?
Phenobarbital's label (effective August 27, 2026) and its 1 other brand-application label do not name nelfinavir 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
| Nelfinavir | Phenobarbital | |
|---|---|---|
| Labeled for | HIV | Seizures and epilepsy |
| Boxed warning | No | No |
| Pharmacy cost (NADAC) | n/a | $0.213 per tablet 100 mg tablet |
| Shortage now | No current entry | 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