For decades, many African women in need of birth control they could use in secret have relied on intramuscular hormone injections that prevent pregnancy for three month s .

But in recent years, women have been terrified — and family planning officials frustrated — as studies suggested that women using injectables were far more likely to get infected with H.I.V.

On Thursday, a major new study found that women who did were not at a much greater risk than they were from other contraceptive methods, including a hormone implant or a copper intrauterine device.The World Health Organization will view the study next month as it debates whether to give back the injectable its top safety rating. Two years ago, the W.H.O. lowered its rating one notch, but said the benefits still outweighed the risks.

The hormone is known as DMPA, for intramuscular depot medroxyprogesterone acetate, and is marketed by Pfizer under the brand name Depo-Provera.