Women using the Pill or other hormonal contraceptives are roughly twice as likely to contract HIV or pass on the AIDS virus to a partner, according to a study published on Tuesday.
The research was carried out among 3,790 heterosexual couples in Africa where one partner had the human immunodeficiency virus (HIV) while the other was uninfected.
The findings, if confirmed, could have huge repercussions for policies on contraception and HIV prevention.
The authors say it strengthens the need for safe-sex messages, in which the condom is promoted as a shield against the AIDS pathogen.
The couples were monitored for an average of 18 months during which 167 individuals became infected, 73 of them women, according to the study published in The Lancet Infectious Diseases.
Transcribed into a benchmark of prevalence, HIV transmissions were 6.61 per 100 person-years in couples where women used hormonal contraception, compared to 3.78 per 100 person-years among those who did not.
Rates of infection from women to men were 2.61 per 100 person-years among women who used hormonal contraception, but 1.51 per 100 person-years among those who did not.
Most of the women who took part in the study used injectable, long-lasting contraceptive forms such as the Depo-Provera shot. Only a small number used the Pill. In the latter group, there was an increase in HIV risk but not big enough to be conclusive.
Over the last two decades, scientists have launched several investigations into whether hormonal contraceptive use affects HIV risk, but the probes have returned conflicting results.
This is the first large-scale study to return clear proof of the risk.
The investigators noted that women who took injectable contraceptives had "raised concentrations" of HIV genetic material in their cervical secretions.
The study, led by Renee Heffron of the University of Washington, suggests that doctors advise women of the potential increased risk and warn them of "dual protection" with condoms.
The study was conducted between 2004 and 2010 in Botswana, Kenya, Rwanda, South Africa, Tanzania, Uganda and Zambia as part of a trial of a therapy against the herpes simplex virus, which is common among people with HIV.
In 2009, more than 33 million people were living with HIV and 2.6 million people became newly infected, according to figures released last year by UNAIDS.
