What the FDA labels say about taking them together
Efavirenz and phenytoin
Efavirenz's FDA label of March 31, 2024 names phenytoin in its warnings and precautions section, and phenytoin's label names efavirenz too. The efavirenz label: “Patients who are receiving concomitant anticonvulsant medications primarily metabolized by the liver, such as phenytoin and phenobarbital, may require periodic monitoring of plasma levels.”
Check it at the source: the efavirenz FDA label on DailyMed (effective March 31, 2024, Cipla USA Inc.); the phenytoin FDA label on DailyMed (effective October 15, 2024, Mylan Pharmaceuticals Inc.).
What does the efavirenz label say about phenytoin?
Efavirenz label, warnings and precautionsPatients who are receiving concomitant anticonvulsant medications primarily metabolized by the liver, such as phenytoin and phenobarbital, may require periodic monitoring of plasma levels [see Drug Interactions ( 7.1 ) ].
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 phenytoin label say about efavirenz?
Phenytoin 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 (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
| Efavirenz | Phenytoin | |
|---|---|---|
| Labeled for | HIV | Seizures and epilepsy |
| Boxed warning | No | No |
| Pharmacy cost (NADAC) | $1.28 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