What the FDA labels say about taking them together
Phenytoin and ritonavir
Phenytoin's FDA label of October 15, 2024 names ritonavir in its drug interactions section, and ritonavir's label names phenytoin too. The ritonavir label: “A dose increase may be needed for these drugs when co-administered with ritonavir and therapeutic concentration monitoring is recommended for these anticonvulsants, if available.”
Check it at the source: the phenytoin FDA label on DailyMed (effective October 15, 2024, Mylan Pharmaceuticals Inc.); the ritonavir FDA label on DailyMed (effective July 23, 2026, AbbVie Inc.).
What does the phenytoin label say about ritonavir?
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
What does the ritonavir label say about phenytoin?
Ritonavir label, drug interactionsConcomitant Drug Class/Drug Name: Anticonvulsants: divalproex, lamotrigine, phenytoin; Effect on Concentration of Ritonavir or Concomitant Drug: ↓ anticonvulsants; Clinical Comment: A dose increase may be needed for these drugs when co-administered with ritonavir and therapeutic concentration monitoring is recommended for these anticonvulsants, if available.
Source: FDA drug label (Norvir), AbbVie Inc., effective July 23, 2026 (SPL set 2849298e-de6e-47bb-8194-56e075b33fc3), 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
| Phenytoin | Ritonavir | |
|---|---|---|
| Labeled for | Seizures and epilepsy | HIV |
| Boxed warning | No | Yes |
| Pharmacy cost (NADAC) | $0.145 per capsule 100 mg capsule | $0.607 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