A man in his twenties has contracted a rare strain of HIV while taking Truvada as pre-exposure prophylaxis (PrEP).

Howard A. Grossman, MD presented the findings at the HIV Research for Prevention (HIVR4P) in Chicago, which ends Friday.

According to POZ.com, the man tested negative for HIV before starting PrEP on January 1 of this year. He was diagnosed with a rare, highly drug-resistant strain of HIV on May 3. Tests indicate that he was adhering to the daily PrEP regimen.

The man said that he had unprotective sex with two men outside of his relationship with his HIV-positive partner before testing positive. In both sexual encounters, the man played the top.

POZ.com noted that such cases are rare.

“PrEP works and is the most successful intervention we have had; failures have been vanishingly rare, and that's not changing; and tops can get infected,” Grossman told POZ.com. He added that condoms can help reduce the risk of infection.

This is the second case of a person taking PrEP as prescribed contracting HIV. The first case was presented in February.