WWW- Use of the contraceptive Depo Provera appears to triple women’s risk of infection with chlamydia and gonorrhea, a study reports Monday.

An estimated 20 million to 30 million women worldwide use Depo Provera, which is injected into the arm or buttocks every three months.

“It’s popular among young women particularly,” says Christine Mauck of the Contraceptive Research and Development Program in Arlington, Va. Not only is it convenient and effective, says Mauck, who wasn’t involved in the new study, “it can’t be found by your mother.”

But other studies have suggested that Depo Provera, as well as oral contraceptives, raise users’ risk of contracting chlamydia and gonorrhea, two common sexually transmitted diseases.

The study, which appears in the journal Sexually Transmitted Diseases, focused on 819 women ages 15 to 45 who were just starting birth control prescribed at two Baltimore-area Planned Parenthood (news – web sites) clinics. About three-quarters were single. Of the women, 354 chose the pill, 114 chose Depo Provera and 351 opted for a non-hormonal contraceptive. The women were tested for chlamydia and gonorrhea after three, six and 12 months.

By the end, 45 women had contracted chlamydia or gonorrhea. Women using Depo Provera were about three and a half times more likely to develop one of the infections than women using non-hormonal contraceptives. The researchers say they can’t yet explain their finding.

They also found that pill users were 50% more likely to become infected than users of non-hormonal contraceptives, but there were so few cases that could have been due to chance, says lead author Charles Morrison of Family Health International in Research Triangle Park, N.C.

Morrison says one or two more high-quality studies are needed to confirm his findings. But, he says, the study does highlight the need for hormonal contraceptive users to also use condoms if they aren’t in mutually monogamous relationships. Hormonal contraceptives alone don’t protect against STDs, and, as this study suggests, Depo Provera might raise the risk of infection.

Because researchers didn’t randomly assign women to contraceptive methods, they can’t be absolutely sure whether the Depo Provera itself or some characteristic of women who opted for it raised the infection risk, Mauck says. But the authors say it’s unlikely that differences in the women led to the finding.

The study was paid for by the National Institute of Child Health and Human Development and the U.S. Agency for International Development