Bone mineral density (BMD) is a measurement of the amount of calcium and other minerals contained in a section of bone. This density is a primary indicator of bone strength and a major determinant of a woman’s lifetime risk for osteoporosis and fragility fractures. Given that combined oral contraceptive pills (COCs), often referred to simply as “the pill,” are a widely used form of birth control, the relationship between these medications and bone health is a significant public health question. Understanding how these synthetic hormones interact with the skeletal system is particularly relevant as women may use COCs for many years, including during the critical period of peak bone mass accrual.
The Core Finding: Do Oral Contraceptives Affect Bone Density
Combined oral contraceptives generally do not result in a loss of bone density in women who have already achieved their peak bone mass. For most adult users, the effect of COCs on existing BMD is considered neutral or sometimes even slightly positive. Studies often show that bone density in long-term adult COC users is comparable to that of women who have never used the pill. The estrogen component in COCs is thought to help maintain bone mineral density in mature women.
It is important to distinguish COCs from other hormonal birth control methods, particularly the injectable contraceptive depot medroxyprogesterone acetate (DMPA). DMPA has been consistently associated with a temporary decrease in BMD across all age groups. In contrast, COCs, which contain both a synthetic estrogen and a progestin, do not carry the same risk of bone loss for the average adult user.
How Hormones Regulate Bone Metabolism
Bone tissue is constantly being remodeled through bone formation by cells called osteoblasts and bone resorption by osteoclasts. Natural, or endogenous, estrogen plays a fundamental role in this process by suppressing the activity of osteoclasts, helping to maintain bone mass and strength. This hormonal influence is why estrogen deficiency, such as that experienced after menopause, accelerates bone loss.
Combined oral contraceptives introduce synthetic hormones, including a form of estrogen, that interact with this natural system. While the synthetic estrogen is bone-protective, the COCs also suppress the body’s natural ovarian hormone production. This suppression results in a lower overall level of endogenous estrogen and a blunting of the body’s natural bone turnover rate. This altered hormonal environment can sometimes slightly suppress the optimal rate of bone accrual, rather than causing outright bone loss. Changes in BMD are precisely measured using Dual-Energy X-ray Absorptiometry (DEXA) scans.
Impact Based on Age of Use and Duration
The timing of COC use relative to skeletal development is the most important factor affecting bone density. Peak bone mass (PBM), the maximum amount of bone a person will achieve, is established during adolescence and early adulthood, typically by the mid-twenties. Using COCs during this critical window of bone development presents a unique consideration.
Studies show that adolescents and young adults who start COCs may experience a smaller gain in BMD compared to their non-using peers. This reduced rate of accrual is a concern because a lower PBM can increase the lifetime risk of fracture. This effect has been observed even with low-dose formulations, and it appears to be most pronounced when COCs are started within the first three years following the start of menstruation.
For women who have already reached PBM, COCs primarily function in a bone maintenance role. In this group, the medication is not generally associated with a detrimental effect on existing bone density. Furthermore, research suggests that any small deficit in PBM accrual observed in adolescents may be reversible or stabilize after the pill is discontinued.
Mitigating Risks and Maintaining Bone Health
Maintaining bone health involves several lifestyle factors that are important regardless of contraceptive choice. Ensuring an adequate daily intake of bone-building nutrients is necessary. Young women should aim for a daily calcium intake of approximately 1,000 to 1,200 milligrams, and a Vitamin D intake of 600 to 800 International Units (IU).
Regular weight-bearing exercise is also fundamental, as activities like running, dancing, or weightlifting stimulate bone formation and strength. Avoiding habits that are toxic to the skeletal system, such as smoking and excessive alcohol consumption, is necessary to preserve bone mass. Consuming sufficient dairy products, for example, has been shown to protect the hip and spine BMD from the temporary reduction sometimes observed in young women using COCs.
Women with pre-existing risk factors for low bone density, such as a family history of osteoporosis, very low body mass index, or a history of previous fractures, should discuss these with a healthcare professional. A provider can determine the most appropriate contraceptive choice and monitoring plan.