What the FDA labels say about taking them together
Efavirenz and fosphenytoin
Fosphenytoin's FDA label of May 18, 2025 names efavirenz in its drug interactions section; the efavirenz label PlainMeds reads does not name fosphenytoin. The fosphenytoin label: “Interacting Agent: Antiviral agents; Examples: Efavirenz, lopinavir/ritonavir, indinavir, nelfinavir, ritonavir, saquinavir Fosamprenavir: phenytoin when given with fosamprenavir alone may decrease the concentration of …”
Check it at the source: the efavirenz FDA label on DailyMed (effective March 31, 2024, Cipla USA Inc.); the fosphenytoin FDA label on DailyMed (effective May 18, 2025, Pfizer Laboratories Div Pfizer Inc).
What does the efavirenz label say about fosphenytoin?
Efavirenz's label (effective March 31, 2024) does not name fosphenytoin in these sections. A label lists what its studies and reports found; it is not a complete list.
Source: FDA drug label (Efavirenz), Cipla USA Inc., effective March 31, 2024 (SPL set 92603fa5-7a2c-4bfb-9a70-aadb4fba952c), via openFDA; the full label on DailyMed
What does the fosphenytoin label say about efavirenz?
Fosphenytoin label, drug interactionsInteracting Agent: Antiviral agents; Examples: Efavirenz, lopinavir/ritonavir, indinavir, nelfinavir, ritonavir, saquinavir Fosamprenavir: phenytoin when given with fosamprenavir alone may decrease the concentration of amprenavir, the active metabolite. Phenytoin when given with the combination of fosamprenavir and ritonavir may increase the concentration of amprenavir
Source: FDA drug label (CEREBYX), NDA020450 (Parke Davis), as published by Pfizer Laboratories Div Pfizer Inc, effective May 18, 2025 (SPL set de5a6711-4abb-4c3d-a18b-266b094ace11), 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
| Efavirenz | Fosphenytoin | |
|---|---|---|
| Labeled for | HIV | Seizures and epilepsy |
| Boxed warning | No | Yes |
| Pharmacy cost (NADAC) | $1.28 per tablet 600 mg tablet | n/a |
| 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