What the FDA labels say about taking them together
Efavirenz and phenobarbital
Efavirenz's FDA label of March 31, 2024 names phenobarbital in its drug interactions section; the phenobarbital labels PlainMeds reads (2) do not name efavirenz. 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 phenobarbital FDA label on DailyMed (effective August 27, 2026).
What does the efavirenz label say about phenobarbital?
Efavirenz label, drug interactionsDrugs to Affect Efavirenz Drugs that induce CYP3A activity (e.g., phenobarbital, rifampin, rifabutin) would be expected to increase the clearance of efavirenz resulting in lowered plasma concentrations [ see Dosage and Administration ( 2.2 ) ].
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 phenobarbital label say about efavirenz?
Phenobarbital's label (effective August 27, 2026) and its 1 other brand-application label do not name efavirenz in these sections. A label lists what its studies and reports found; it is not a complete list.
Source: FDA drug label (Phenobarbital), Chartwell RX, LLC, effective August 27, 2026 (SPL set 74428144-fb24-40a5-a670-3b876e2d116c), 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 | Phenobarbital | |
|---|---|---|
| Labeled for | HIV | Seizures and epilepsy |
| Boxed warning | No | No |
| Pharmacy cost (NADAC) | $1.28 per tablet 600 mg tablet | $0.213 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