From the Globe and Mail:
Bone loss could be permanent, contraceptive maker warns
Women using the injectable contraceptive Depo-Provera can lose significant bone mineral density, damage that may be irreversible and puts them at increased risk of developing osteoporosis, the drug's maker is warning.

In a letter being sent to health professionals who prescribe the drug, Dr. Bernard Prigent, medical director of Pfizer Canada Inc., said "bone loss is greater with increasing duration and may not be completely reversible."