US doctors from the Johns Hopkins School of Hygiene and Public Health in Baltimore said a study of 650 drug users showed that women needed 50% less of the virus to go on to develop Aids.

They said their research showed the need for a change in clinical guidelines on drug treatments for Aids.

They added that the guidelines appeared to have been biased towards a white homosexual male model of the disease.

The researchers, led by Dr Homayoon Farzadegan, do not know the reason women require less of the virus to develop Aids.

Immune system

People can carry HIV for several years without becoming ill. Some people have never proceeded to Aids. The virus attacks the immune system.

A person is said to have Aids when the immune system becomes so compromised that they begin to develop a series of infections which they find difficult to shake off.

These can include pneumonia, a form of skin cancer called Kaposi's sarcoma and dementia.

It has been known for some years that women are more likely to become infected with HIV than men.

This is because of the biology of sex and because HIV is difficult to contract and has to be pushed into the body.

New combinations of drugs used to treat HIV have been very successful in halting and reversing the progress of the disease, although they do not work for all patients.

The US researchers say there may be biological reasons why women may need to take the drugs earlier than men.

Writing in The Lancet magazine, Dr Farzadegan said: "The possibility that women, according to current guidelines, will be undertreated on a population level must be carefully considered."

'Neglected'

A spokesman for the National Aids Trust (NAT) in the UK said the research showed the need for more studies into the effect of HIV on women.

Testing for HIV: women are more likely to be infected and more likely to get Aids

He said the majority of people infected with HIV in the UK continued to be gay men, which would explain the bias in research.

However, he added that this was not the case in the rest of the world, particularly in developing countries, where women were the most affected group.

Even in the US and other countries in Europe, the prevalence of intravenous drug users has meant more women have become infected with HIV.

"Women have been neglected in research, even though they are more vulnerable in the first place," he said.

The NAT and Positively Women are to approach the British HIV Association, the body of Aids experts who influence clinical policy on the disease, to discuss the need for more research on women and drug treatment in the light of the American study.

Call for more research

Elizabeth Crafer, director of the Positively Women charity, said: "These findings highlight the lack of comprehensive research we have about how HIV affects women.

"We urgently need to see work in the UK on whether new drug treatments should be prescribed earlier for women than men in order to help prolong more lives."

She added that women, particularly African women - the most affected heterosexual group in the UK - needed to be involved in the research.

The organisation believes women do not often come forward for research into HIV. This may be for many reasons, including concerns about the effect of drugs on the reproductive system.

It says little research has been done into the effect of combination drug therapy on women, including its side effects.