Hormone shots that are a popular form of birth control in Africa may increase the risk of HIV infection for women who use them and the men who are their sexual partners.

The risk for male infection from an HIV-positive female partner appeared to nearly double when hormonal forms of birth control were used versus other methods or none, according to a study just published online in Lancet Infectious Diseases. For women using the contraceptives, the risk of HIV infection from an infected male partner also nearly doubled.

Both hormonal contraceptives and the pill were used by women in the study, conducted in East and Southern Africa between 2004 and 2010. But the increased risk was only significant for the injection. Variations in condom use didn't account for the difference observed.

Previous research had suggested hormonal contraceptives could raise the HIV infection risk for women. This study is the first to show that the risk is heightened for their male partners. More than 3,000 couples participated in the research.

The researchers write that some experiments suggest hormonal contraception could lead to various changes that would increase the infectiousness of HIV, possibly including an increase in the shedding of HIV in the cervix and vagina. But the exact mechanism, if it exists, isn't known.

The findings aren't ironclad proof because the couples weren't randomly assigned to receive hormonal contraceptives or an alternative. Nonetheless, World Health Organization epidemiologist Mary Lyn Gaffield told the New York Times, "We are going to be re-evaluating WHO's clinical recommendations on contraceptive use."

Given the importance of effective birth control and the substantial risk of HIV infection in so many African countries, "the donor community should support a randomized trial of hormonal contraception and HIV acquisition" to settle the matter, an accompanying Lancet editorial said.

The study was funded by the National Institutes of Health and the Bill & Melinda Gates Foundation, which also provides financial support to NPR.