The birth control shot known as Deprovera is an attractive option for women because it can be taken just four times a year, rather than every day as is common with birth control pills. But women receiving the shot should pay extra attention to their bone health, new research finds. Doctors at the University of Texas followed 95 women using Deprovera for two years and periodically assessed their bone strength. Half of users had a sizeable drop in mineral density in their spine or hip bones, which could lead to fracture risk. The shot causes a long-term disturbance in hormones, which both prevents pregnancy and causes bone changes. Past research suggests that the bone strength typically recovers when women stop using Deprovera, but this can't be guaranteed and experts advise women to limit the duration they are on the shot to no more than two years. Smokers run particular risk for bone loss, so women can reduce their odds by not smoking and doctors recommend taking calcium and vitamin D to boost bone strength as well.