Most drugs can be used for more than just the disease they were initially designed to treat.

For example, disulfiram (brand name Antabuse) was developed to treat parasites but has since been found to be useful for treating alcoholism. It’s much quicker and cheaper to ‘repurpose’ drugs in this way than it is to develop them from scratch, which is why we now have dexamethasone — a steroid developed in the late 1950s — for treating severe cases of COVID-19.