Women who use hormonal birth control are roughly twice as likely to become infected with HIV or pass on the AIDS virus to their 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 paper appearing 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 hormone contraceptives used an injectable, long-lasting form such as the Depo-Provera shot. Only a small number used the Pill; in this 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, using an ambitious design, to return clear proof of the risk. It is also the first to highlight an apparent risk to men.
The investigators noted that women who took injectable contraceptives had "raised concentrations" of HIV genetic material in their cervical secretions.
If this is a mechanism for handing on the virus to men, further work is urgently needed to test the theory, they said.
In practical terms, doctors should advise women of the potentially increased risk and warn them of "dual protection" with condoms, says the probe, led by Renee Heffron of the University of Washington in Seattle.
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 a commentary also carried by the journal, clinical scientist Charles Morrison spoke of a "tragic" dilemma.
Promoting hormonal birth control in Africa could be contributing to the HIV epidemic; yet limiting this highly effective form of contraception would also boost rates of maternal death and sickness, underweight babies and orphans.