Molnupiravir Side Effects: What to Expect and When to Call Your Doctor

Molnupiravir (Lagevrio) is generally well tolerated — clinical trials showed that most patients experience few or no significant side effects. But there are important warnings, particularly for pregnant individuals and those who could become pregnant, that every patient should understand before taking this medication.

This guide covers what to expect when taking molnupiravir, what side effects are rare but serious, and exactly when you should call your doctor.

IMPORTANT: Boxed Warning — Fetal Harm

Molnupiravir carries a boxed warning — the FDA's most serious safety alert — regarding potential fetal harm. Based on animal studies, molnupiravir may cause birth defects or embryo-fetal lethality. There are no adequate human studies on use during pregnancy.

If you are pregnant: Molnupiravir is not recommended. It may only be used if your doctor determines the benefits outweigh the risks and there are no other COVID-19 treatment options available.

If you are a woman who could become pregnant: Use reliable contraception during treatment and for 4 days after your last dose.

If you are a man with a partner who could become pregnant: Use reliable contraception during treatment and for at least 3 months after your last dose.

Breastfeeding: Do not breastfeed during treatment or for 4 days after your last dose. Pump and discard breast milk during this time.

Common Side Effects of Molnupiravir

In the Phase 3 MOVe-OUT clinical trial, the most commonly reported side effects of molnupiravir were mild. Most patients tolerated the drug well. The most common side effects include:

Diarrhea — the most commonly reported GI effect; usually mild and self-limiting.

Nausea — taking the capsules with food may help reduce stomach upset.

Dizziness — avoid driving or operating heavy machinery if you experience dizziness.

These effects were reported at similar rates in both the molnupiravir and placebo groups in the trial, which suggests some of what patients experience may be symptoms of COVID-19 itself rather than the drug.

Serious Side Effects: What to Watch For

While serious side effects are rare with molnupiravir, you should seek emergency medical help immediately if you experience:

Serious allergic reaction: Hives, difficulty breathing, swelling of the face, lips, tongue, or throat — call 911 immediately.

Worsening COVID symptoms: Increased breathlessness, persistent chest pain, confusion, or an inability to stay awake — seek emergency care.

Unexpected pregnancy: If you or your partner becomes pregnant during treatment or within 3 months after treatment (males), contact your provider immediately and enroll in Merck's pregnancy registry (1-877-888-4231).

What About Bone and Cartilage Effects?

Animal studies showed bone and cartilage toxicity with repeated dosing of molnupiravir. This is why the drug is not authorized for patients under 18 — developing bone and cartilage may be more vulnerable. In adult humans taking the standard 5-day course, bone toxicity has not been reported as a clinical concern. However, this was the key reason the FDA did not authorize its use in children or adolescents.

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Drug Interactions: The Good News

One of molnupiravir's major advantages is that no clinically significant drug-drug interactions have been identified. This is a key difference from Paxlovid, which interacts with dozens of commonly prescribed medications. While you should always tell your provider about all medications you take, molnupiravir is generally safe to take alongside other drugs.

Alcohol and Molnupiravir

There are no known interactions between alcohol and molnupiravir. However, heavy alcohol consumption suppresses immune function and may worsen your COVID-19 illness. Additionally, alcohol may increase dizziness, which is already a potential side effect. It is best to avoid alcohol while you are sick with COVID-19, regardless of what antiviral you are taking.

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When to Call Your Doctor During Treatment

Call your provider if:

Your COVID-19 symptoms are getting significantly worse, not better, after 2–3 days of treatment.

You experience severe diarrhea or dehydration.

You develop any signs of allergic reaction (rash, hives, swelling).

You discover or suspect you are pregnant.

You miss a dose and are unsure what to do.

What Happens If You Miss a Dose?

If you miss a dose:

If it has been less than 10 hours since your scheduled dose, take it as soon as you remember.

If it has been more than 10 hours, skip the missed dose and take your next dose at the regular time.

Never take two doses at once to make up for a missed one.

Additional Resources

For information on drug interactions, read our guide on Molnupiravir Drug Interactions: What to Avoid. If you're having trouble finding molnupiravir, medfinder.com helps locate pharmacies with it in stock near you.