COVID-19 Antiviral — RNA Polymerase Mutagenic Agent Pregnancy: Absolute contraindication — mutagenic in animal studies; teratogenic and embryotoxic. MHRA absolutely contraindicated. If inadvertently taken in early pregnancy — refer to teratology information service and obstetrics urgently.

Molnupiravir

Brand names: Lagevrio

Adult dose

Dose: 800 mg (4 × 200 mg capsules) twice daily for 5 days

Route: Oral

Frequency: Twice daily for 5 days

Max: 1600 mg/day

COVID-19 in adults with at least one risk factor for severe disease. Start within 5 days of symptom onset. NOT recommended as first-line when Paxlovid (nirmatrelvir/ritonavir) can be used — MHRA and NICE preference is Paxlovid first. Molnupiravir reserved for patients where Paxlovid is contraindicated due to drug interactions or renal impairment. Source: BNF 90; NICE TA871.

Paediatric dose

Dose: Not licensed under 18 years N/A/kg

Route: N/A

Frequency: N/A

Max: N/A

Not licensed in paediatric COVID-19 — mutagenicity concern particularly relevant in growing children.

Dose adjustments

Renal

No dose adjustment required for renal impairment — key advantage over Paxlovid (can use in eGFR <30 mL/min where Paxlovid is contraindicated).

Hepatic

No dose adjustment required — hepatic metabolism is not a primary route.

Paediatric weight-based calculator

Patient weight (kg)

Not licensed in paediatric COVID-19 — mutagenicity concern particularly relevant in growing children.

Clinical pearls

MOVe-OUT trial (NEJM 2022): molnupiravir reduced hospitalisation or death by 30% vs placebo in high-risk unvaccinated COVID-19 patients — significantly less effective than Paxlovid (89%). This explains MHRA/NICE hierarchy: Paxlovid first-line; molnupiravir second-line when Paxlovid cannot be used.
Why Paxlovid is preferred: Paxlovid reduces hospitalisation by 89% vs 30% for molnupiravir. Molnupiravir's advantage is the ABSENCE of drug interactions (no ritonavir) — making it appropriate for patients on anticoagulants, tacrolimus, statins, or other drugs contraindicated with ritonavir, and for patients with eGFR <30 mL/min.
Mutagenic mechanism — the reproductive concern: molnupiravir is a nucleoside analogue that causes lethal mutagenesis of SARS-CoV-2. It introduces errors into the viral RNA polymerase. However, molnupiravir and its active metabolite NHC can potentially be incorporated into host DNA, raising theoretical mutagenicity concern. Animal studies showed skeletal abnormalities in offspring — hence absolute pregnancy CI and contraception requirement in men for 3 months (spermatogenesis cycle).
Contraception counselling is mandatory: women of childbearing potential must use effective contraception DURING treatment and for 4 days after the last dose. Male patients with fertile female partners: use contraception during treatment and for 3 months after. This is an MHRA licence condition — document counselling.
Timing is critical: start within 5 days of symptom onset. Efficacy at days 6–10 is markedly lower — viral replication has peaked by day 4–5. MHRA REMS requires eligibility assessment and prescription within 5-day window. Early testing and rapid prescribing pathway essential. Source: BNF 90; Jayk-Bernal et al. NEJM 2022 (MOVe-OUT); NICE TA871; MHRA SPC Lagevrio.

Contraindications

Pregnancy — absolute contraindication (mutagenic mechanism — see pearls)
Breastfeeding (mutagenic — do not use; hold breastfeeding during treatment and for 4 days after final dose)
Women of childbearing potential without reliable contraception during treatment
Men who are partners of women who could become pregnant must use reliable contraception during treatment and for 3 months after final dose

Side effects

Diarrhoea, nausea, dizziness, headache (most common — generally mild and self-limiting)
Mutagenicity theoretical concern (mechanism of action — see pearls)
Rash, pruritus (less common)

Interactions

No significant CYP-based drug interactions (unlike Paxlovid ritonavir) — this is the primary advantage of molnupiravir over Paxlovid for patients on complex medication regimens
Uridine: co-administration may reduce molnupiravir efficacy (substrate competition) — avoid uridine supplements

Monitoring

Symptom diary (COVID-19 course — expected improvement by day 3–4)
Pregnancy test before prescribing in women of childbearing potential (mandatory MHRA requirement)
Contraceptive history and counselling documented at prescribing
Symptom worsening monitoring — if deteriorating, reassess for hospitalisation

Reference: BNFc; BNF 90; Jayk-Bernal et al. NEJM 2022 (MOVe-OUT); NICE TA871; MHRA SPC Lagevrio. Verify against your local formulary and the latest BNF before prescribing.

Curated clinical cross-links plus same-class fallbacks.

Calculators

Pathways