A new study found that using the contraceptive Depo Provera appears to triple women’s risk of chlamydia and gonorrhea infection. An estimated 20 million-30 million women use the contraceptive, which is injected into the arm or buttocks every three months.

"It’s popular among young women particularly," said Christine Mauck of the Contraceptive Research and Development Program in Arlington, Va. Mauck said it is convenient, effective, and "it can’t be found by your mother." Mauck was not involved in the research.

The study focused on 819 women ages 15-45 who were just starting birth control prescribed at two Baltimore-area Planned Parenthood clinics. Nearly three-quarters were single. Among respondents, 354 chose the birth-control pill, 114 chose Depo Provera, and 351 picked a non-hormonal contraceptive. Clinic personnel tested the women for chlamydia and gonorrhea at 3-, 6- and 12-month intervals.

By the end of the study, 45 women had contracted chlamydia or gonorrhea. Women using Depo Provera were about 3.5 times more likely to contract the infections than women using non-hormonal contraceptives. Researchers cannot explain their findings.

The investigators also found that pill users were 50 percent more likely to become infected than women using non-hormonal contraceptives, but there were so few cases that the results could have been due to chance, said lead author Charles Morrison of Family Health International in Research Triangle Park, N.C.

Morrison said one or two more high-quality studies are needed to confirm the present report’s findings. The study highlights the need for hormonal contraceptive users to use condoms as well if they are not in mutually monogamous relationships, Morrison said.

Because researchers did not randomly assign women to contraceptive methods, they cannot be absolutely sure whether Depo Provera itself or some characteristic of women who opted for it accounts for the raised infection risk, Mauck said. But the authors said it is unlikely that differences in the women led to their finding.

The study, "Hormonal Contraceptive Use, Cervical Ectopy, and the Acquisition of Cervical Infections," appeared in Sexually Transmitted Diseases (2004;31(9):561-567).