Some researchers worry the drug may create more contagious or health-threatening variations of Covid, which has killed more than 6.8 million people globally over the past three years. Mutations linked to the use of Merck's pill, Lagevrio, have been identified in viral samples taken from dozens of patients, according to a preprint study from researchers in the US and at the Francis Crick Institute, Imperial College London and other UK institutions. The drug-linked mutations of the virus haven't been shown to be more immune-evasive or lethal yet, according to the study published Friday without peer review on the medRxiv website. But their very existence highlights what some scientists say are potential risks in wider use of the drug, which was recently cleared in China.