What the FDA labels say about taking them together
Nelfinavir and phenytoin
Nelfinavir's FDA label of December 11, 2025 names phenytoin in its drug interactions section, and phenytoin's label names nelfinavir too. The nelfinavir label: “… nelfinavir plasma concentrations in patients taking these agents concomitantly. Phenytoin plasma/serum concentrations should be monitored; …”
Check it at the source: the nelfinavir FDA label on DailyMed (effective December 11, 2025, AGOURON); the phenytoin FDA label on DailyMed (effective October 15, 2024, Mylan Pharmaceuticals Inc.).
What does the nelfinavir label say about phenytoin?
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 phenytoin label say about nelfinavir?
Phenytoin label, drug interactionsInteracting Agent: Antiviral agents; Examples: Fosamprenavir, nelfinavir, ritonavir
Source: FDA drug label (Phenytek), Mylan Pharmaceuticals Inc., effective October 15, 2024 (SPL set 0f94c791-8b07-4b41-927e-c9f28bec5f60), 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 | Phenytoin | |
|---|---|---|
| Labeled for | HIV | Seizures and epilepsy |
| Boxed warning | No | No |
| Pharmacy cost (NADAC) | n/a | $0.145 per capsule 100 mg capsule |
| Shortage now | No current entry | No current entry |
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