Taking antiretrovirals may stave off HIV infection

The drugs might stop the virus getting established, even if a non-infected person is exposed.

However, there are some fears it might discourage the use of condoms - or cause long-term side-effects.

The researchers, from the Royal Free Hospital in London, are calling for studies to test their theory.

Pregnant women with HIV are often given a short course of anti-HIV drugs before the birth of the baby to minimise the chance of the virus being passed from mother to child.

However, the idea of giving such drugs, even in low doses, to adults, has never been fully investigated.

Risky strategy

The only strategies offered to HIV-positive men or women are the use of barrier contraceptives such as condoms, or abstinence.

These are not 100% effective, even when used correctly, and the possibility of condom failure, as well as the temptation not to use protection, can increase the chance of HIV transmission.

We would not be putting out the message that you should not use condoms. That would be completely irresponsible Dr Mike Youle, Royal Free Hospital

New HIV cases, particularly among heterosexuals, are increasing in the UK despite continued public health campaigns, and the extension of life offered by anti-HIV drugs means that many more people are likely to embark on relationships with people who do not have the virus.

Dr Mike Youle told the BBC: "I would feel that if we had agents that could treat HIV, they would also be likely to prevent HIV and this should be evaluated.

"There are animal studies that show you can prevent infection with these drugs."

He said that people taking the drugs would have take one tablet a day, or possibly one just before having sex.

All protection possible

Dr Youle said he would still encourage them to use a condom.

He said: "We would targeting in the initial studies people who had already expressed the fact that they were not using condoms regularly with their infected partner.

We should be looking at the rights of people who are in HIV relationships or with HIV to be thinking about whether this gives them the opportunity to be having condomless sex that many others enjoy Will Nutland, Terrence Higgins Trust

"We would not be putting out the message that you should not use condoms. That would be completely irresponsible."

Will Nutland, from the Terrence Higgins Trust, told the BBC that the charity would be "broadly supportive" of more research.

He said: "I'm not worried this could compromise a safe sex message.

"Preventative treatment is another tool in the range of prevention measures - some use risk reduction strategies."

He said that comparisons could be drawn between the introduction of the contraceptive pill in the 1960s and the possibilities offered by an HIV preventing drug.