What the FDA labels say about taking them together
Letermovir and thioridazine
Letermovir's FDA label of January 14, 2026 names thioridazine in its drug interactions section; the thioridazine label PlainMeds reads does not name letermovir. The letermovir label: “Co-administration of PREVYMIS and thioridazine 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 thioridazine FDA label on DailyMed (effective November 11, 2025, Mylan Institutional Inc.).
What does the letermovir label say about thioridazine?
Letermovir label, drug interactionsConcomitant Drug Class and/or Clearance Pathway/Drug Name: thioridazine; Effect on Concentration ↓ =decrease, ↑ =increase: ↓ letermovir; Clinical Comments: Co-administration of PREVYMIS and thioridazine 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 thioridazine label say about letermovir?
Thioridazine's label (effective November 11, 2025) does 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 (Thioridazine Hydrochloride), Mylan Institutional Inc., effective November 11, 2025 (SPL set 52fea941-0b47-41c1-b00d-f88150e8ab93), 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 | Thioridazine | |
|---|---|---|
| Labeled for | n/a | n/a |
| Boxed warning | No | Yes |
| Pharmacy cost (NADAC) | n/a | $0.571 per tablet 25 mg tablet |
| 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