Women who receive the birth control shot may be at a higher risk of contracting HIV, reveals a meta-analysis of 12 studies at the University of California, Berkeley.

The research, published Thursday in the journal The Lancet Infectious Diseases, considered 39,500 women in sub-Saharan Africa who received depot medroxyprogesterone acetate— a shot sold under the name Depo-Provera. In their review, the study authors adjusted their data to account for condom usage, according to a news release. 

Compared to women who used other, non-hormonal forms of birth control or women who didn’t use birth control at all, women who used Depo-Provera had a 40 percent increased risk of contracting HIV.

According to 2010 data from the Guttmacher Institute, a nonprofit that works to advance reproductive health, about 61,750 women in the United States use some form of birth control. About 2.4 percent, or 1,450 women, use an injectable form of contraceptive, including the birth control shot. The shot protects against pregnancy for three months, and of every 100 women who get the shot, about six will get pregnant, according to the U.S. Department of Health & Human Services. 

The most commonly used injectable contraceptive is Depo-Provera, sometimes called Depo.

The studies in the review allowed researchers to weigh HIV infection risk among different types of hormonal contraceptives. In addition to Depo, study authors also analyzed whether an elevated HIV risk existed with use of hormonal contraceptives, including norethisterone oenanthate, sold as NET-EN, combined oral contraceptives, and progestin-only pills.

They noted that use of the other methods wasn’t linked to an increased HIV risk. Study authors hypothesized that birth control with higher levels of progestin may have changed the vaginal lining or altered local immunity, increasing the risk of HIV infection. However, they said more research on birth control’s biological and physiological effects is needed.

About 144 million women worldwide use hormonal contraception, and nearly 41 million of them use injectable forms of birth control rather than the pill.

“The results have potentially broad implications because hormonal contraceptives remain popular for women worldwide,” study lead author Lauren Ralph, who did this research for her dissertation in epidemiology at UC Berkeley, said in the news release.

Researchers cautioned that their results didn’t suggest that most women who use this form of birth control withdraw from it— especially in sub-Saharan Africa, where the study results were drawn.

“It can be tricky to ensure a reliable supply of contraceptives in sub-Saharan Africa,” Ralph said. “Removing Depo-Provera doesn’t mean the women will have immediate access to other methods of birth control that are as effective. Ultimately, decisions around which birth control method to use should be made between a woman and her healthcare provider.”

The study authors said further study may focus on high-risk populations, as well as the development of new contraceptive methods.

“The most important next steps for women all over the world are to examine ways to broaden women’s contraceptive options and increase uptake of other safe and effective contraceptive methods, and to step up research on new contraceptive methods, especially those that protect against both HIV and pregnancy,” senior author Nancy Padian, a UC Berkeley adjunct professor of epidemiology, said in the news release.