Women in the study who used hormonal contraception had double the risk ofacquiring HIV or transmitting it to their male partners as those who did not use hormonal contraception.

While hormonal conception includes both oral contraception and injectable forms of birth control, the findings were most pronounced for women using injectables, likeDepo-Provera, the study said.

"These findings have important implications for family planning and HIV-1 prevention programs, especially in settings with high HIV-1 prevalence", said study researcher Jared Baeten, of the University of Washington. HIV-1 is the prevalent subtype of HIV.

"Recommendations regarding contraceptive use, particularly emphasizing the importance of dual protection with condoms and the use of non-hormonal and low-dose hormonal methods for women with or at risk for HIV-1, are urgently needed", said study researcher Renee Heffron, also of the University of Washington.

HIV and contraception

More than 140 million women worldwide use hormonal contraception, such as daily oral pills and long-acting injectables, the study said. A large proportion of the 16 million women living with HIV in sub-Saharan Africa also use hormonal contraception.

Additionally, women who were HIV-positive at the beginning of the study and using injectable contraception were twice as likely to transmit the virus to their male partner as women who did not use hormonal contraception.

The results held even after researchers took into account factors that could affect the HIV transmission rate, such as the whether the woman was pregnant, and whether condoms were used.

It's possible hormonal contraception causes biological changes, such as changes to the cells that line the vagina or cervix, that influence susceptibility to HIV, the researchers said.

What's to be done?

"Active promotion of [injectable contraceptives] in areas with high HIV incidence could be contributing to the HIV epidemic in sub-Saharan Africa, which would be tragic," Charles Morrison from Clinical Sciences, Durham, North Carolina, said in an accompanying editorial.

"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 birth weight babies and orphans — an equally tragic result," Morrison said.

Morrison also noted the study was not originally designed to measre the effect of hormonal contraception on HIV risk, and that the number of women using these contraceptives was small. In addition, it was common for women in the study to switch their contraception method, from hormonal to non-hormonal contraception, such as intrauterine devices.

It's time to find a definitive answer to the question of whether hormonal contraception increases HIV acquisition risk, Morrison said. This can be done through a trial in which participants are randomly assigned to receive hormonal contraception or not.

• 10 Medical Myths That Just Won’t Go Away
• Top 10 Mysterious Diseases
• 7 Diseases You Can Learn About From a Genetic Test