What the FDA labels say about taking them together
Fentanyl and letermovir
Letermovir's FDA label of January 14, 2026 names fentanyl in its drug interactions section; the fentanyl labels PlainMeds reads (2) do not name letermovir. The letermovir label: “When PREVYMIS is co-administered with a CYP3A substrate, refer to the prescribing information for dosing of the CYP3A substrate with a moderate CYP3A inhibitor. …”
Check it at the source: the fentanyl FDA label on DailyMed (effective March 6, 2026, Hikma Pharmaceuticals USA Inc.); the letermovir FDA label on DailyMed (effective January 14, 2026, Merck Sharp & Dohme LLC).
What does the fentanyl label say about letermovir?
Fentanyl's label (effective March 6, 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 (Fentanyl Citrate), Hikma Pharmaceuticals USA Inc., effective March 6, 2026 (SPL set ce5db1c2-feb1-4ad2-847a-d02e865bd47e), via openFDA; the full label on DailyMed
What does the letermovir label say about fentanyl?
Letermovir label, drug interactionsConcomitant Drug Class and/or Clearance Pathway/Drug Name: Examples: alfentanil, fentanyl, midazolam, and quinidine; Effect on Concentration ↓ =decrease, ↑ =increase: ↑ CYP3A substrate; Clinical Comments: When PREVYMIS is co-administered with a CYP3A substrate, refer to the prescribing information for dosing of the CYP3A substrate with a moderate CYP3A inhibitor. When PREVYMIS is co-administered with cyclosporine, the combined effect on CYP3A substrates may be similar to a strong CYP3A inhibitor. Refer to the prescribing information for dosing of the CYP3A substrate with a strong CYP3A inhibitor. CYP3A substrates pimozide and ergot alkaloids are contraindicated [see ].
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
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
| Fentanyl | Letermovir | |
|---|---|---|
| Labeled for | Pain | n/a |
| Boxed warning | Yes | No |
| Pharmacy cost (NADAC) | $1.79 per mL 100 mcg/2 mL vial | n/a |
| Shortage now | In shortage | 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