In The Lancet paper, the researchers looked at monkeypox cases in the U.K. from August 2018 to September 2021 and identified seven patients who were given two antiviral drugs used to treat smallpox: brincidofovir and tecovirimat. While these two drugs are approved for use in the U.S. under special circumstances like instances of bioterrorism involving smallpox, their efficacy has only been tested in animals. (It’s against ethical rules to intentionally infect humans with smallpox for the sake of a clinical trial, Dr. Stanley Deresinski, an infectious disease specialist at Stanford University who wasn’t involved in the study, told The Daily Beast.)

Out of the two drugs, tecovirimat seemed to come out as the clear winner. Three patients with monkeypox who were on brincidofovir all developed elevated liver enzymes, which is a sign the liver is distressed or getting wrecked. But the one patient on tecovirimat seemed to tolerate the antiviral drug well enough and only spent 10 days in the hospital compared to the others who spent up to two to three weeks, even more than a month.