Inertia progesterone IMPROVES bone density. That's why they give you it in HRT to help reverse osteopirosis after the menopause. The difference in bone mass using or not using progesterone-based contraceptives in women who had not yet reached peak bone mass (teenagers and early 20's) was found to be statistically insignificant. So as far as bones go, Mirena is safe. In addition it protects one from endometrial cancer.

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