What the FDA labels say about taking them together
Cobicistat and phenytoin
Cobicistat's FDA label of February 10, 2026 names phenytoin in its contraindications section; the phenytoin labels PlainMeds reads (2) do not name cobicistat. The cobicistat label: “Coadministration with carbamazepine, phenobarbital, or phenytoin is contraindicated due to potential for loss of therapeutic effect and development of resistance.”
Check it at the source: the cobicistat FDA label on DailyMed (effective February 10, 2026, Gilead Sciences, Inc.); the phenytoin FDA label on DailyMed (effective October 15, 2024, Mylan Pharmaceuticals Inc.).
What does the cobicistat label say about phenytoin?
Cobicistat label, contraindicationsAlpha 1-adrenoreceptor antagonist: alfuzosin Antianginal: ranolazine Antiarrhythmic: dronedarone Anticonvulsants: carbamazepine, phenobarbital, phenytoin Anti-gout: colchicine Antimycobacterial: rifampin Antineoplastics: irinotecan These contraindications apply only to TYBOST coadministered with atazanavir Antipsychotics: lurasidone, pimozide Ergot Derivatives: dihydroergotamine, ergotamine, methylergonovine Herbal Products:
Cobicistat label, drug interactionsConcomitant Drug Class/Drug Name: Anticonvulsants: carbamazepine, phenobarbital, phenytoin; Potential Effect ↑ = Increase, ↓ = Decrease, ↔ = No change: ↓ atazanavir ↓ darunavir ↓ cobicistat; Clinical Comment: Coadministration with carbamazepine, phenobarbital, or phenytoin is contraindicated due to potential for loss of therapeutic effect and development of resistance.
Source: FDA drug label (Tybost), Gilead Sciences, Inc., effective February 10, 2026 (SPL set 3784c35c-e87f-410c-900b-8fd6313c6010), via openFDA; the full label on DailyMed
What does the phenytoin label say about cobicistat?
Phenytoin's label (effective October 15, 2024) and its 1 other brand-application label do not name cobicistat 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
| Cobicistat | 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 | A presentation being discontinued | 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