Because the role of the drugs as a preventive measure has been concealed in case it encouraged unsafe sex and triggered a rush for expensive drugs - some doctors have claimed that the treatment is not being made available to that need it .
Studies have shown that in pregnant women with HIV and people jabbed with contaminated needles, antiviral drugs given promptly can stop transmission of the disease, either to the baby or the injured person. Scientists believe that prophylactic treatment following unprotected intercourse with an HIV-infected person can also restrict transmission, although no studies have been done.
AZT, the Aids drug can reduce the risk of transmission to the baby from 26 per cent to 8 per cent when given to pregnant women with HIV before and during delivery, and to the infant for six weeks after birth. In people jabbed with used needles, AZT treatment for three to four weeks afterwards reduces the risk of infection by 80 per cent. Guidelines in the US recommend that health workers who suffer needlestick injuries should be treated with a cocktail of three drugs for four weeks afterwards.
A review of Aids drugs in the Drug and Therapeutics Bulletin, a guide for doctors published by the Consumers' Association, says prophylactic treatment should be offered to HIV-infected pregnant women to prevent infection of their babies and to people injured by contaminated needles. But it says there is no agreement on whether people should get it after unsafe sex.
Dr Joe Collier, editor of the Bulletin said: "Prophylactic treatment after unsafe sex should be thought through carefully and shouldn't be overlooked. There are unusual circumstances - such as rape - where it could be appropriate but people don't know about it."
Some doctors in the UK had already prescribed the drugs to patients after risky sex and a study of their efficacy as a preventive measure following unprotected intercourse was beginning in San Francisco, Dr Collier said. "If you get a scratch with an infected needle it is crucial you get down to a clinic quickly and get treatment. It can reduce the incidence a lot. The same is true for HIV-infected pregnant women.
"For people exposed to the virus through sex, it is more difficult. Aids doctors don't want people phoning up every morning after they have had unsafe sex. That is an abuse of the arrangements when they should be wearing condoms or having safe sex."
Professor Michael Adler, head of the Aids clinic at University College Hospital, London, said most hospitals now had a policy of treating pregnant women and health workers who suffered needlestick injuries but there was concern at the development of a "morning after" mentality in relation to sex.
"These drugs are toxic and four weeks' treatment costs pounds 600. If people feel there is a morning after pill and they don't have to have safe sex it sends the wrong message."
n The Aids virus is becoming more aggressive and dangerous, causing recently infected people to become ill faster, according to a study. Doctors in Italy who studied 285 patients infected between 1985 and 1995 found that those infected during the past six years were more likely to have immune system cells knocked out by the virus, and developed Aids more quickly. However, British experts said they had seen no similar change in the course of the disease in the UK.