Pill and bone density: good evidence only near menopause

Bone density is built and defended largely through the reproductive years, and oestrogen is one of its main regulators, which is why hormonal preparations have long been proposed as a way to protect the skeleton. The question is complicated because the relevant groups are so different: a healthy woman on the pill, a woman whose periods have stopped because of low energy availability or heavy training, and a woman approaching menopause are not in the same hormonal position. Assuming a benefit shown in one group applies to the others is a common mistake. This review set out to sort what the evidence actually supported in each.

The authors reviewed 75 articles on the effect of oral contraceptives and other forms of hormone replacement on bone density in premenopausal and perimenopausal women. Rather than pooling the numbers statistically, they appraised each body of evidence using the Oxford Centre for Evidence-Based Medicine levels of evidence, then graded the strength of the conclusion for each population separately. Groups considered included perimenopausal women, premenopausal women with hypothalamic oligo- or amenorrhoea, healthy premenopausal women and anorexic premenopausal women. Baseline bone density in the different groups was also considered as part of judging how clinically important treatment decisions were.

The strongest conclusion was for perimenopausal women, where the review found good evidence of a positive effect of oral contraceptives on bone density. Evidence was rated fair for a positive effect in premenopausal women with hypothalamic oligo- or amenorrhoea, and only limited in healthy premenopausal women and in anorexic premenopausal women. The review also noted that baseline bone density in hypothalamic oligo- or amenorrhoeic women has been shown to be significantly lower than in healthy controls, which makes the decision to treat clinically more consequential in that group. What the review could not determine was the ideal formulation or the ideal duration of treatment.

This is a graded map of a literature rather than a single answer, and its main contribution is showing that the evidence thins out sharply as you move away from the perimenopausal group. The stratification helps explain why claims about the pill and bone health conflict so often: different studies were asking about different populations with different starting bone density. Key gaps the authors name are formulation and duration, both of which need longitudinal and prospective randomised trials in larger samples. The review is also from 2006, so it does not incorporate anything published since.

If bone density is a concern for you, this is a reason to think the strength of the hormonal argument depends heavily on which stage of life you are at, with the case rated strongest around perimenopause. For women whose periods have become irregular or stopped for hypothalamic reasons, the review flags that bone density tends to start lower, making the clinical stakes higher — a point to raise with a clinician rather than act on alone. For healthy premenopausal women, the evidence was rated limited, so the pill should not be assumed to be a bone-protection strategy. Which formulation and how long remain genuinely open questions.