This past week, the Food and Drug Administration (FDA) authorized two different versions of an antiviral pill for Covid treatment at home, Paxlovid, from Pfizer, and molnupiravir, from Merck. And while both pills can reduce your risk of serious disease or death from Covid if you’re unvaccinated, the FDA says the Merck Covid pill should not be used if you’re currently pregnant or might become pregnant.

Both pills work to fight off severe disease from Covid if taken within 5 days of showing symptoms. And while the Merck version is expected to be more widely available, Pfizer’s Paxlovid appears to be more effective: reducing risk of hospitalization and death in unvaccinated patients by 88%. Merck’s molupiravir has been shown in clinical trials to reduce the same risk factors by just 30%, making it less effective. And also riskier, in terms of reproductive health.

How the Merck Covid pill could affect pregnancy

The Merck pill works by inserting errors into the virus’ DNA to stop its genes from replicating. Because it does affect DNA, using the Merck pill could be dangerous for pregnant women or those trying to get pregnant—as it could cause unintended consequences in rapidly dividing cells, potentially causing birth defects or even fetal death.

To clarify, pregnant and breastfeeding individuals and women who could likely become pregnant were not included in Merck’s clinical trials, so these risks have not yet been seen in humans. However, an expert reported to The New York Times that Merck did report that high doses of the medication in pregnant rats could cause developmental abnormalities or fetal death in the drug’s animal trials. Experts are concerned about similar effects happening in human adults. Scientists are urging Merck to publish its data from its rodent trials.

The Merck Covid pill is authorized only for high-risk adults

Currently, the FDA has authorized the Merck treatment for high-risk adults only, and gone so far as to say it should not be a preferred treatment for Covid, considering it is much less effective than Paxlovid. The Merck Covid pill has been cleared for use in adults who are at high risk of severe disease from Covid and for whom other Covid treatment options are not available or appropriate. 

The FDA states that women who are pregnant should not take the pills, but that there may be exceptions to this. If women who may become pregnant take the pills, they should use contraception while taking the drugs and for at least four days after the last dose. Male partners of women who could get pregnant should use contraception while taking the Merck pills and for at least three months after. 

Molnupiravir is not authorized for use in people younger than 18, because it may affect bone and cartilage growth. 

Paxlovid does not insert genetic mutations, so it is considered much safer by many health experts. However, the FDA states the decision to take Paxlovid should be discussed with your healthcare provider if you are currently pregnant or breastfeeding. 

Neither Paxlovid nor molnupiravir should be used as a way to prevent Covid, before or after potential Covid exposure. The best method to prevent contracting the virus, including the Omicron variant is by getting fully vaccinated, which includes two doses and one booster shot of an mRNA vaccine, and which has not been shown to have any harmful reproductive effects.