Does vitamin D reduce menstrual pain, or does this review overstate the benefit?
The review suggests that oral vitamin D may reduce pain in people with primary dysmenorrhoea—menstrual pain without an identified underlying pelvic condition—over two to four menstrual cycles. Across nine trials listed in the full report, the authors report a standardised pain reduction of 1.40 standard deviations compared with control, with a 95% confidence interval from 0.73 to 2.08. This is not a 1.40-point reduction on a ten-point pain scale. The magnitude remains uncertain because trial results differed substantially. Some comparisons added vitamin D to painkillers or calcium, so the findings do not establish that vitamin D can replace established treatment.
Why is the reported average pain reduction difficult to rely on?
This provisional full-text assessment identifies substantial uncertainty despite the useful focus on randomised trials and patient-reported pain. Between-study heterogeneity was very high: the reported I² of 92% indicates that the results varied substantially beyond expected sampling variation. One trial reported an exceptionally large benefit, while another did not clearly favour vitamin D. The authors also detected funnel-plot asymmetry, which raises—but does not prove—the possibility that small-study effects or selective publication exaggerated benefit. Blinding was incomplete in some trials. Together, these issues make the pooled average an uncertain guide to the improvement an individual patient might experience.
Does this review establish an effective and safe vitamin D dosing schedule?
No particular schedule is established as optimal or adequately characterised for safety. Both higher- and lower-dose groups showed pooled pain reductions; separate subgroup findings do not demonstrate that higher doses work better. Moreover, the narrative misattributes lower-dose duration results to higher-dose treatment, and one interval cited as supporting pain relief includes no effect. These confirmed reporting inconsistencies weaken the dosing conclusions. Although the authors state that the included trials did not identify the overdose-related harms discussed, there is no pooled harms estimate or detailed safety assessment. Short follow-up therefore cannot establish the safety of repeated high doses or justify the proposed 300,000-IU loading regimen.
Assessed 18 Sept 2026.
