What the FDA labels say about taking them together
Letermovir and phenobarbital
Letermovir's FDA label of January 14, 2026 names phenobarbital in its drug interactions section; the phenobarbital labels PlainMeds reads (2) do not name letermovir. The letermovir label: “Co-administration of PREVYMIS and phenobarbital is not recommended due to potential for loss of efficacy of PREVYMIS.”
Check it at the source: the letermovir FDA label on DailyMed (effective January 14, 2026, Merck Sharp & Dohme LLC); the phenobarbital FDA label on DailyMed (effective August 27, 2026).
What does the letermovir label say about phenobarbital?
Letermovir label, drug interactionsConcomitant Drug Class and/or Clearance Pathway/Drug Name: phenobarbital; Effect on Concentration ↓ =decrease, ↑ =increase: ↓ letermovir; Clinical Comments: Co-administration of PREVYMIS and phenobarbital is not recommended due to potential for loss of efficacy of PREVYMIS.
Source: FDA drug label (PREVYMIS), Merck Sharp & Dohme LLC, effective January 14, 2026 (SPL set 1b49df80-be4f-47e0-a0b7-123f3e69395b), via openFDA; the full label on DailyMed
What does the phenobarbital label say about letermovir?
Phenobarbital's label (effective August 27, 2026) and its 1 other brand-application label do not name letermovir 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
| Letermovir | Phenobarbital | |
|---|---|---|
| Labeled for | n/a | Seizures and epilepsy |
| Boxed warning | No | No |
| Pharmacy cost (NADAC) | n/a | $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