A study indicates that a drug used to treat HIV after infection can also lower the risk of gay men becoming infected with the virus.

The research, published this week in the New England Journal of Medicine, found that giving Truvada to gay men at high risk of HIV lowered their chances of acquiring the virus by 40 per cent.

Truvada is currently used as an antiretroviral combination in people who are already infected with HIV but the study suggests it could be used before infection, as Pre-Exposure Prophylaxis (PrEP).

Data from this trial, called iPrEx, found an estimated 43.8 per cent reduction of new HIV infections among men who took Truvada daily to prevent HIV, compared to those who took a placebo pill.

The research studied 2,499 men in Peru, Ecuador, Brazil, South Africa, Thailand and the US. Their average age was 27 and all were at a high risk of HIV infection through risky sexual behaviour.

The men were given condoms and safe sex counselling. Over 18 months, 100 became HIV-positive. As 36 were in the Truvada group and 64 were in the placebo group, it was estimated that this equated to a 44 per cent reduction in infection.

Although most of the men said they took the dose almost every day, blood testing showed that many had not. However, among those who did take a daily dose, the risk of infection fell by 95 per cent.

HIV experts greeted the findings with excitement, although they warned that there was no evidence to suggest that they would be replicated among other high-risk groups and that the obligation to take a pill daily could be a barrier to treatment.

Terrence Higgins Trust’s chief executive, Sir Nick Partridge, said the trial results were “potentially very significant” but warned that, for now, condoms remain “the most effective, easily available and cheapest way of preventing HIV transmission”.

He added: “Three major hurdles are still going to be its cost, the risks of drug-resistant strains of HIV developing and taking a drug treatment every day. There is already a trial underway assessing the intermittent use of anti-HIV drugs before and after having sex and we expect the results of that trial in the next few months.

“For now, and for the foreseeable future, condoms remain the most effective, easily available and cheapest way of preventing HIV transmission. As this trial suggests, Pre-Exposure Prophylaxis is going to be an addition to condom use rather than a replacement of it.”