A University of Washington study on 3,800 African couples in which one partner was already infected with HIV has found that an injectable contraceptive seems to double the risk that women who take it will be infected with HIV. Also, when HIV-positive women used this method their male partners were twice as likely to become infected.

The New York Times reports that about 12 million women between 15-49 in sub-Sarahan Africa use this injectable hormone contraception, as opposed to 1.2 million women in the U.S.

The biological effects would likely be the same for all women, but scientists “emphasized that concern was greatest in Africa because the risk of H.I.V. transmission from heterosexual sex was so much higher there than elsewhere,” writes the Times.

The World Health Organization will meet in January to review the research and decide whether there is enough evidence to warn women that this method may increase their HIV risks (getting and transmitting), “and the possibility (not examined in the new study) that hormonal contraceptives accelerate H.I.V.’s severity in infected women.”

The branded version of the injectable is Depo-Provera, though the African women were probably using generic versions. Oral contraceptives also appeared to increase HIV risk but there were too few pill users in the study to be “statistically significant.” The study also showed that pregnancy may also double the risk of contracting or transmitting HIV, which could be related to either hormones or unprotected sex.