A Sydney man has been diagnosed with HIV despite taking a "game-changing" pre-exposure prophylaxis medication known as PrEP. Steven Spencer, 27, tested positive for HIV in December, despite using PrEP "on demand" before and after sexual encounters in line with the advice of doctors.

"I was in complete shock, as were my doctors," Mr Spencer said, knowing the chances were extremely rare.

Roughly half a million people worldwide take the medication, which is more than 99 per cent effective in preventing HIV when taken correctly. The sexual health and gay community advocate has chosen to share his story to prevent misinformation that could devalue the effectiveness of PrEP.

"What happened to me doesn’t change the fact that PrEP is still the most powerful HIV preventative we have ever had," said Mr Spencer, one of the first men in Australia to start taking the medication more than five years ago. Read more via Sydney Morning Herald