A new study released Monday said Merck‘s widely used antiviral Covid pill can cause mutations in the virus that occasionally spread to other people, raising questions about whether the drug has the potential to accelerate Covid’s evolution. 

The findings may increase scrutiny about the usefulness of the treatment, molnupiravir, which was one of the first Covid drugs available to doctors worldwide during the pandemic.

Molnupiravir works by causing mutations in Covid’s genetic information, which weakens or destroys the virus and reduces the amount of Covid in the body. However, the study published Monday in the scientific journal Nature found that Covid can sometimes survive treatment with molnupiravir, leading to mutated versions of the virus that have been found to spread to other patients. 

Researchers in the U.S. and U.K. specifically analyzed 15 million Covid genomes to see which mutations had occurred and when. They found that mutations increased in 2022 after molnupiravir was introduced in many countries. 

There is no evidence that molnupiravir, sold under the brand name Lagevrio, has produced more transmissible or severe variants of Covid, according to the study. 

But the findings are important for regulators who continue to assess the risks and benefits of molnupiravir, wrote Theo Sanderson, the lead author of the study and a researcher at the Francis Crick Institute in London, in a post on X, formerly Twitter.

A spokesperson for Merck pushed back on the new study, claiming the researchers assumed that the mutations they analyzed were associated with molnupiravir-treated patients “without documented evidence of that transmission.”

“Instead, the authors rely on circumstantial associations between the region from which the sequence was identified and timeframe of sequence collection in countries where molnupiravir is available to draw their conclusion,” the spokesperson said.

The spokesperson added that genomes with the mutations were “uncommon and were associated with sporadic cases.” 

The company in February also disputed an earlier study by the same team of researchers, which suggested that molnupiravir is giving rise to new mutations of the virus in some patients. Based on data at the time, a spokesperson for Merck said it didn’t believe molnupiravir was likely to contribute to Covid mutations.

The new study comes as Covid once again gains a stronger foothold in the U.S., primarily driven by newer strains of the virus.

But the U.S. and other countries appear to be relying less on molnupiravir to fend off Covid this year: Sales of the drug dropped to around $200 million during Merck’s third quarter, down 83% from the more than $1 billion reported during the same period a year ago. 

Merck’s molnupiravir has long been controversial because of its ability to cause genetic mutations. 

The U.S. Food and Drug Administration first approved the drug for emergency use in late 2021. But the FDA recommends against using Lagevrio during pregnancy because non-clinical studies suggest that it may cause fetal harm.

Molnupiravir also isn’t authorized for use in patients under 18 because it may affect bone and cartilage growth.