Many Olympic athletes are taking advantage of a loophole that allows them to take a legal prescription drug that may improve their performance, but has not been banned by anti-doping authorities, NPR reports. Athletes in sports including tennis, swimming, cycling and volleyball are practicing this “legal doping.”

“If it’s not banned, athletes will use it,” said Ronald Evans, Director of the Gene Expression Laboratory at the Salk Institute and an investigator for the Howard Hughes Medical Institute.

One such drug is telmisartan, which is sold as a blood pressure medication. The drug, which can improve blood flow, is being monitored by the World Anti-Doping Agency, but has not yet been prohibited.