Merck has claimed the pill developed by Pfizer to treat Covid-19 will create problems for some patients taking drugs for other medical conditions.

Last week, a panel of external experts at the FDA voted narrowly in favour of giving the green light to Merck’s drug by a margin of 13 to 10. But some members raised concerns over its effectiveness and safety, including whether it could cause birth defects if taken by pregnant women, and whether it could potentially help spawn new Covid variants.

The pharmaceutical company’s criticism of a competing product comes after the US FDA considers whether to authorise Merck’s antiviral drug molnupiravir, the first easy-to-administer oral treatment for Covid.

Eliav Barr, senior vice-president of global medical affairs at Merck told the Financial Times both antiviral drugs were needed to protect people from the virus, as some patients may not be able to take one treatment because of possible side effects.

As Barr said, “Ritonavir, which has been used to treat HIV for many years, is a drug that specifically inhibits the body’s ability to break down drugs. The problem is that it’s incredibly non-specific. So there’s a whole host of medicines that people take, especially those medicines, unfortunately, that are associated with conditions that confer risk. There is a whole army of drugs that people are taking, especially drugs that are unfortunately associated with dangerous diseases.”

A study published in the scientific journal Nature last week identified an “extremely high frequency” of drug interactions in treated Spanish patients with Covid-19 who were taking the antiviral drug lopinavir with ritonavir.