As the omicron surge pummels a pandemic-weary nation, the first antiviral pills for Covid-19 promise desperately needed protection for people at risk of severe disease. However, many people prescribed Pfizer’s or Merck’s new medications will require careful monitoring by doctors and pharmacists, and the antivirals may not be safe for everyone, experts caution.

The Food and Drug Administration authorized Pfizer’s Paxlovid for mild to moderate Covid in people as young as 12 who have underlying conditions that raise the risk of hospitalization and death from the coronavirus, such as heart disease or diabetes. However, one of the two drugs in the antiviral cocktail could cause severe or life-threatening interactions with widely used medications, including statins, blood thinners and some antidepressants. And the FDA does not recommend Paxlovid for people with severe kidney or liver disease.

Because of experts’ concerns about the potential side effects of Merck’s molnupiravir, the FDA has restricted its use to adults and only in scenarios in which other authorized treatments, including monoclonal antibodies, are inaccessible or are not “clinically appropriate.”

The Paxlovid cocktail consists of two tablets of the antiviral nirmatrelvir and one tablet of ritonavir, a drug that has long been used as what is known as a boosting agent in HIV regimens. Ritonavir suppresses a key liver enzyme called CYP3A, which metabolizes many medications, including nirmatrelvir. In the case of Paxlovid treatment, ritonavir slows the body’s breakdown of the active antiviral and helps it remain at a therapeutic level for longer.