Truvada was originally developed to fight the virus in HIV-positive people, but now we know HIV-negative people can use it to prevent infection if they are exposed to the virus. The U.S. Food and Drug Administration currently approves Truvada as PrEP as do the equivalent government bodies in Peru, France, South Africa, Kenya, Israel, and Canada.
There are other countries like Thailand and Brazil, where PrEP is available to some people through clinical trials. And in many more places, activists are demanding access to PrEP but their goals are not yet realized.
Check out PrEP Watch for more on individual countries.