Scientists are developing a pill to be taken before sex that would stop transmission of HIV. In the latest development in the battle against the global epidemic which claimed two million lives last year, researchers are investigating whether drugs used to treat the disease might be harnessed to protect against it.

Antiretroviral drugs are already used to prevent transmission from mothers to their babies during birth and scientists hope the same protection can be obtained during sex. If successful, the research would raise ethical questions about the prevention of HIV, including who should take such a drug, in what circumstances and with what risk of side effects.

Three trials of antiretroviral drugs which are being given to uninfected people at high risk of HIV are under way around the world and are "showing great promise", according to a report in The Lancet, published to co-incide with the International Conference on Aids in Mexico City.

One British researcher said yesterday that the return of the bathhouse culture among gay and bisexual men, involving sex with multiple partners, could provide the scenario in which a preventive pill might be taken.

Sheena McCormack, a specialist in HIV prevention and reader in clinical epidemiology at Imperial College London, said: "The party scene involving multiple sexual partners is definitely back in London and probably in most European cities. There is metrosexual mixing involving gay, bisexual and some heterosexual cases. We estimate new HIV infections in gay men in London are running at 3 per cent a year".

Her clinic already offers "post-exposure prophylaxis" to people who have had unprotected sex with someone in a high-risk group, involving a month-long course of treatment with three drugs. But a preventive drug would provide an extra option. "People could pop a pill on a Friday night and be covered for the whole weekend," she said.

On a global scale, use of a preventive pill would have to be restricted to groups at highest risk, such as commercial sex workers or injecting drug users, who would take it daily for the duration of their exposure. Concerns about side effects and the development of resistant strains of HIV would first have to be overcome.

The research is being driven by the lack of progress in the search for a vaccine against HIV, and the failure of efforts to develop vaginal microbicides to protect women which has left scientists determined to find any chink in the virus's armour.

"At the moment we are desperate to demonstrate any biological intervention that can prevent HIV that will give us something to work with. The numbers [of HIV-infected people on drugs] are increasing which is fantastic but the burden of treating a growing population of HIV-infected people is bad enough in the UK, and unimaginable for a developing country," Dr McCormack said.

The trials include 2,400 injecting drug users in Thailand, 1,200 heterosexual men and women in Botswana and 3,000 men who have sex with men in five countries. One trial is using tenofovir, a drug used to treat Aids in the West, and two are using Truvada, a combination of tenofovir and STC. A fourth trial involves 980 women in South Africa who are being given an experimental vaginal gel based on tenofovir.

Studies in primates have shown the drugs do prevent transmission, the Lancet report says.

Tenofovir is used to treat Aids, raising worries about resistance if it were also used as a preventive drug. But a new drug called Maraviroc, made by Pfizer, produces high levels of activity in the genital tissue when taken orally and is not used for treatment. It could provide the basis of a future preventive pill.

Clinton's advice to beat Aids: stay faithful

*Bill Clinton made a plea yesterday for a new emphasis on monogamy as a key element in the battle against Aids.

The former US president, not noted for his ability to keep his own marriage vows, said it was very important to change people's attitudes to sex.

In an interview with the BBC recorded in Africa, Mr Clinton said that increasing support for monogamy was not just a problem for the continent worst hit by Aids but for the world.

"To pretend we can ever get hold of this without dealing with that – the idea of unprotected sexual relations with unlimited numbers of partners – I think would be naïve," he said.

Experts believe that the nature of Africans' relationships may help explain the continent's high rate of Aids. Research suggests a higher frequency of overlapping sexual partnerships.

Mr Clinton said the main challenge to fighting Aids in Africa was improving health services, not a lack of money.

"You can get the universal treatment – the money's there now, if we spend it most effectively. But we don't have the health care systems."