A few months ago the FDA
mandated adding a warning
about potential loss of bone density to the Depo-Provera
(a progestin-only birth control shot) label.
Here's a good overview
of the findings of two recent studies about using Depo-Provera
(depot medroxyprogesterone acetate, or DMPA), bone mineral density, and teens. The key points: DMPA suppresses ovulation [the release of the egg from the ovary]; it also reduces ovarian production of estrogen.
Bone mineral density (BMD) declines during DMPA use. However, DMPA use has not been linked to menopausal osteoporosis or fractures.
The transient loss of BMD associated with DMPA use can be attributed entirely to the reduction in ovarian estrogen production.
The transient impact of DMPA on the body's estrogen levels and BMD is similar to trends seen in breastfeeding women.
Recovery of BMD is complete within 12 months after DMPA use is discontinued. Duration of DMPA use was not observed to affect speed of BMD recovery.
Although 'add-back' estrogen supplementation [taking estrogen while using DMPA to compensate for the reduction in the body's estrogen production] prevents BMD loss in DMPA users, the recovery of BMD that occurs after DMPA discontinuation means that the need for such 'add-back' would be unusual.