Researchers gathered this week in Boston documented the first case of an individual contracting HIV while taking Truvada as pre-exposure prophylaxis (PrEP).

David Knox, MD presented the findings at the 2016 Conference on Retroviruses and Opportunistic Infections (CROI), which ended on Thursday.

According to Poz.com, the 43-year-old gay man in the case tested positive for a multi-drug resistant strain of HIV after being on Truvada for two years.

“It is an example that demonstrates that PrEP can sometimes be ineffective in the face of drug resistant virus, in the same way that treatment itself can sometimes be ineffective in the face of drug resistant virus,” said Richard Harrigan, PhD, one of the researchers on the case. “This case demonstrates that while PrEP is beneficial, we can't rely on it to be an infallible magic bullet.”

Gilead's Truvada is a combination of two drugs. The case is the first to document HIV resistance to tenofovir and emtricitabine.