One concern about PrEP is that people will become infected while taking the drugs and that their virus will then develop resistance to Truvada—making those drugs less effective as treatment options. People could also transmit drug-resistant strains to their sex partners.
PrEP studies to date have shown no evidence of HIV drug resistance in people who were HIV-negative when they started taking Truvada and later became HIV-infected. However, HIV drug resistance can occur in people who are already HIV-infected and starting PrEP. Therefore, HIV testing and medical evaluation before starting PrEP are important to help prevent the development of HIV resistance. Experts also recommend that people on PrEP get tested frequently and look out for symptoms of recent HIV infection so that they can stop taking the drugs as soon as possible if they do become infected. Researchers are also conducting demonstration projects to see how and when drug resistance occurs outside of the artificial setting of a randomized clinical trial.