Birth control pills (BCPs) reduce bone density in a dose related relationship, but the effects are small and only develop after two years of use. Long term studies have not been done. A woman's risk of fractures later in life is influenced by the bone mass she gains in her teens through her 20s and this age group has the highest use of oral contraceptives. High dose BCPs taken for 2 years showed a 1-2% less gain in bone density in all areas of the body.
