What the FDA labels say about taking them together
Fostemsavir and phenytoin
Fostemsavir's FDA label of February 13, 2024 names phenytoin in its contraindications section; the phenytoin labels PlainMeds reads (2) do not name fostemsavir. The fostemsavir label: “Coadministration is contraindicated due to potential for loss of therapeutic effect to RUKOBIA.”
Check it at the source: the fostemsavir FDA label on DailyMed (effective February 13, 2024, ViiV Healthcare Company); the phenytoin FDA label on DailyMed (effective October 15, 2024, Mylan Pharmaceuticals Inc.).
What does the fostemsavir label say about phenytoin?
Fostemsavir label, contraindicationsEnzalutamide o Anticonvulsants: Carbamazepine, phenytoin o Antimycobacterial: Rifampin o Antineoplastic:
Fostemsavir label, drug interactionsConcomitant Drug Class/Drug Name: Anticonvulsants: Carbamazepine Phenytoin; Effect on Concentration of Temsavir and/or Concomitant Drug: ↓Temsavir; Clinical Comment: Coadministration is contraindicated due to potential for loss of therapeutic effect to RUKOBIA [see Contraindications ()].
Source: FDA drug label (Rukobia), ViiV Healthcare Company, effective February 13, 2024 (SPL set a21006b7-6d6f-4f06-81b4-17978756452b), via openFDA; the full label on DailyMed
What does the phenytoin label say about fostemsavir?
Phenytoin's label (effective October 15, 2024) and its 1 other brand-application label do not name fostemsavir in these sections. A label lists what its studies and reports found; it is not a complete list.
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
| Fostemsavir | Phenytoin | |
|---|---|---|
| Labeled for | HIV | Seizures and epilepsy |
| Boxed warning | No | No |
| Pharmacy cost (NADAC) | $156.84 per tablet 600 mg tablet | $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