A new study of more than 600 gay and bisexual men showed 100% of them remained HIV-negative after using PrEp for up to two-and-a-half years.

Conducted at San Francisco’s Kaiser Pemanente Medical Center, the study included men who self-reported that they did not use condoms consistently. Their average age was 37, and they reported a higher number of sex partners than those not on PrEP.

“[This] suggests the treatment may prevent new HIV infections even in a high-risk setting,” researcher Jonathan Volk reported in the journal Clinical Infectious Diseases.

Notably, 41% of subjects admitted they used condoms less once they began PrEP, compared to the 56% who said their use of condoms remained consistent.

Thirty percent of participants did, however, contract an STI other than HIV in the first six months of the study. That number jumped 50% to after one year.

"Without a control group, we don’t know if these STI rates were higher than what we would have seen without PrEP,” added co-author Julia Marcus.

"Ongoing screening and treatments for STIs, including hepatitis C, are an essential component," of any prevention program.