U. S. researchers found that about half of women using the birth control shot, known as depot medroxyprogesterone acetate (Depo-Provera), will experience high-bone mineral density loss in the hip or lower spine.

According to a report published in the January issue of Obstetrics & Gynecology, at higher risk were women who had never given birth, who were smokers and who didn't consume much calcium. The losses were experienced in the third year of use also.

Dr Abbey Berenson, professor in the department of obstetrics and gynecology and director of the Center for Interdisciplinary Research in Women's Health at the University of Texas Medical Branch at Galveston, in a university news release said, "Bone mineral density loss is not a significant concern for all women who use this method of birth control.”

In a two year study that included 95 patients who had the injection, 45 women lost at least 5 per cent of bone mineral density in their lower backs and hips. In 27 other participants a research for another year was conducted in which it was found that those at high risk lost even more bone mass.

Not associated with higher bone mineral density loss was age, race or ethnicity, previous contraceptive use and body mass index.