Gay men at high risk of contracting HIV should have access to a daily dose of a drug used to treat the AIDS-causing virus, but as a preventive measure, researchers recommended Thursday.

The medicine, sold under the brand name Truvada, proved almost 90 percent effective at protecting men taking part in a British study, they wrote in The Lancet medical journal.

However, some worry that prescribing it as pre-exposure prophylaxis (PrEP) rather than treatment, would cause a jump in unsafe sex, and with it other sexually transmitted diseases, by removing the fear of contracting HIV.

There is also the prohibitive cost: a year's supply of Truvada, in a western European country, would cost around 7,000 euros ($7,700).

"National health services are under financial constraints, but they cannot afford to ignore the results," wrote the study authors, as they officially published the outcome of the study whose results have already been announced elsewhere.

The findings, they added: "strongly support the addition of PrEP to the current standard of prevention for men who have sex with men at risk of HIV infection."

In the 2012-2014 study, Truvada was given to 544 men who reported having had sex with other men in the previous three months without a condom.

Some were given the drug right away and others after a year -- the first group had an 86-percent lower risk than the second over the study period.

Researchers also observed no significantly higher risk of sexually transmitted diseases like syphilis among the protected group.

"Many gay men are at imminent risk of catching HIV and this is a way we can stop that," study co-author Sheena McCormack of University College London told AFP.

Michael Brady, medical director of the Terrence Higgins Trust, a British anti-HIV/AIDS charity, said the study was "an essential step towards ensuring access to PrEP for those who need it."

The drug has not been universally welcomed in the gay community, eliciting debate about promiscuity, erratic condom use and risks of worsening the stubborn HIV/AIDS epidemic which has killed 36 million people worldwide in the past three decades.

The drug is much less effective if not taken daily, some point out, which means forgetfulness could put your life at risk.