Only a small proportion of gay and bisexual men who could have taken a daily pill to reduce their risk of human immunodeficiency virus (HIV) actually did so in 2014, according to a new study.
Furthermore, young gay and bisexual men and those of colour, who are among the groups most at risk for infection with HIV, were less likely to be taking the medication than their older and white counterparts.
Unequal use of the drug, called Truvada, may worsen the disparities between races in HIV rates, said lead author Henry Raymond, of the San Francisco Department of Public Health.
"We don't want to leave anyone behind," he told Reuters Health.
Truvada, manufactured by Gilead, is a combination of two anti-retroviral drugs that work to keep HIV, which causes AIDS, from replicating in the body. Approved by the US Food and Drug Administration in 2012, Truvada is often just referred to as PrEP, which stands for pre-exposure prophylaxis.
Gay and bisexual men who regularly take Truvada reduce their risk of HIV by 90 per cent, according the US Centres for Disease Control and Prevention (CDC).
For the new study, reported in the journal Sexually Transmitted Infections, Raymond's team analysed data from 411 gay and bisexual men living in San Francisco in 2014.
About three-fourths of the men said they were HIV-negative and of those, 64 per cent met one of the CDC's criteria for Truvada use. Those criteria are having an HIV-positive partner, not being in a monogamous relationship with an HIV-negative partner, having anal sex without a condom or having a sexually transmitted infection in the past six months.
But while nearly two-thirds were eligible for Truvada, only about 14 per cent of those eligible men said they had used it at some point in the past year.
When the researchers applied the study's results to the wider population of gay and bisexual men in San Francisco, they estimated that 27,745 men met the CDC's guidelines for Truvada but weren't using the drug.
About 80 per cent of young gay and bisexual men between ages 18 and 24 met one of those criteria, compared to about 29 per cent of men 55 years old or older, the study found. Yet, none of the younger men reported using Truvada.
About 23 per cent of white gay and bisexual men who met the CDC's criteria took the pill, compared to about four per cent of Hispanic men, seven per cent of Asian men and eight per cent of black men, the researchers found.