OC-related drop in bone density varies with age, hormone dose

Oral contraceptives (OCs) may decrease bone density, but the effect, which is small and appears only after about 2 years of use, depends on the age of the user and the hormone dose in the pill, a new study reports. 

The prospective cohort study, published online July 13 in the Journal of Clinical Endocrinology and Metabolism, evaluated hip, spine, and whole-body bone density every 6 months for 2 to 3 years in 606 women: 301 adolescents between 14 and 18 years of age and 305 young adults between 19 and 30 years of age. Researchers compared the bone densities of the 389 OC users in the group with those of the 217 nonusers. During the study, 172 users stopped taking OCs. After 24 months, adolescents using OCs with 30 to 35 μg of ethinyl estradiol (EE), but not lower doses, showed about 1% less bone density gain than nonusers at the spine (1.32% vs 2.26%) and whole body (1.45% vs 2.3%). Young adult OC users and nonusers showed no differences in bone density at any site.

Researchers also measured bone density changes in the 172 women who discontinued OCs. Adolescents who took pills with 30 to 35 μg of EE still showed smaller bone density gains at the spine than nonusers 12 to 24 months after discontinuing the medication. Young adults showed small losses in spinal bone density 12 to 24 months after stopping both higher- and lower-dose pills, whereas nonusers showed small gains.

The clinical significance of the study findings for fracture risk isn’t known, say the researchers. They advocate studies of longer trends in bone density after discontinuation of OCs to clarify the relationship between OC use and fracture risk.

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