Women who use hormonal methods of birth control such as the pill or the contraceptive injection double their chances both of contracting HIV and passing it onto a male partner, according to a report by Lancet Infectious Diseases.
Scientists think that the risk of contracting HIV doubles because of the changes that progesterone and oestrogen cause to vaginal structure and the immune system.
The hormones in the contraception may also affect proteins, called cytokines, which are released by cells of the immune system and help to regulate it.
Progesterone and Oestrogen could affect a protein called CCR5, which HIV targets in order to enter the body, although this is a very new area of research.
Nearly four thousand African couples were studied tto produce the report, of which the vast majority used the contraceptive injection. Of the 16 million women living with HIV in sub-Saharan Africa, many use hormonal contraception, making it important to verify previously inconsistent reports.
It is difficult to weigh the benefit of these findings with the reality of life in Africa, according to Charles S Morrison from Clinical Sciences Durham USA.
“Active promotion of [the contraceptive injection] in areas with high HIV incidence could be contributing to the HIV epidemic in sub-Saharan Africa, which would be tragic," he said.
“Conversely, limiting one of the most highly used effective methods of contraception in sub-Saharan Africa would probably contribute to increased maternal mortality and morbidity and more low birthweight babies and orphans—an equally tragic result.
“The time to provide a more definitive answer to this critical public health question is now”
In the UK, Genevieve Edwards, Director of Health Promotion for HIV and sexual health charity Terrence Higgins Trust, said:
"These findings reinforce the message that hormonal contraception, while generally effective against unwanted pregnancy, will not protect against sexually transmitted infections, including HIV. Too many people think the Pill will protect them from infection. It won't.
"Even if you are taking other forms of contraception, we recommend using condoms unless you're in a relationship and both of you have attended a full sexual health screening."
Rebecca Findlay, from the sexual health charityFamily Planning Association (FPA) supported this message, saying:
“All women, whatever their method of contraception need to be confident and happy about using condoms as its estimated about a quarter of people with HIV haven’t been diagnosed.”