Well PrEP may not be as safe as we thought.
There's a case of a man contracting HIV while using the drug. Scientists did say this was possible, it was never completely, completely safe. However, cases like this one will remain pretty rare.
Here is so more info
David Knox, MD, an HIV specialist at the Maple Leaf Medical Clinic and the lead author of the case study, presented findings at the 2016 Conference on Retroviruses and Opportunistic Infections (CROI) in Boston. Evidence suggests that the individual in question, a 43-year-old man who has sex with men, adhered well to PrEP over the long-term. Nevertheless, after 24 months on Truvada he tested positive for HIV. Initial tests indicated that he was acutely (very recently) infected: He tested positive for the p24 antigen, which appears within about three weeks of HIV infection and disappears a few weeks afterward; and at that time he tested negative for HIV antibodies, which typically appear two to eight weeks after infectionGo here for more on this issue, and if you're on PrEP, please play safe reguardless