Use of the contraceptive Depo Provera appears to triple women’s risk of infection with chlamydia and gonorrhea, according to a study released today.

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

Other studies also have suggested Depo Provera – as well oral contraceptives – raises users’ risk of contracting chlamydia and gonorrhea, two common sexually transmitted diseases.

The new study, reported in the journal Sexually Transmitted Diseases, focused on 819 women 15 to 45 years old who were just starting birth control prescribed at two Baltimore area Planned Parenthood clinics. About three-quarters were single.

By the end, 45 women had contracted chlamydia or gonorrhea. But 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 findings.

They also found that pill users were 50 percent more likely to become infected than users of non-hormonal contraceptives, but there were so few cases that itcould have been due to chance.