Women on Deprovera take just four shots a year, versus taking common birth control pills daily.
Doctors at the University of Texas followed 95 patients using Deprovera for two years. They made periodic checks on the patients' bones and found half of them had sizable decreases in the mineral density of the spine and hips, which can increase the risk of fractures.
Experts involved in the study are advising women currently taking Deprovera to limit the duration they are on the shots to no more than two years.
The study is published in the latest Obstetrics and Gynecology.