Background Bacterial vaginosis (BV) is the most common cause of vaginal infection worldwide and is associated with myriad negative reproductive health outcomes. Several cross-sectional studies indicate that women with low vitamin D levels have increased BV prevalence.

Methods This randomised, double-blinded, placebo-controlled trial started enrollment in September 2011 and concluded follow-up in January 2013. Women (n = 126) with symptomatic BV were enrolled from an urban STD clinic in the midwestern United States. All participants received standard metronidazole therapy. Intervention participants (n = 63) also received nine doses of 50,000 international units of cholecalciferol (vitamin D3) over 6 months; control arm women (n = 63) received matching placebo. BV status was assessed via Nugent scoring at three follow-up visits over six months. The primary analysis will be intention-to-treat using extended Cox proportional hazard models.

Results Participants’ median age was 26. Three-quarters (75%) of women were black and 25% were white. All reported a lifetime history of sex with men, and 29% also had a lifetime history of sex with women. At baseline, median serum vitamin D levels (measured as 25 -hydroxy vitamin D) were the same for intervention and control women at 15.85 ng/mL (interquartile range (IQR): 12.1–21.4 ng/mL); levels < 20 ng/mL are considered insufficient. Eight-one percent of participants returned for one or more follow-up visits. At trial completion, median vitamin D level among intervention women was 30.5 ng/mL (IQR 24.4–37.7 ng/mL), vs. 17.8 ng/mL among control women (IQR: 11.7–27.1 ng/mL). Nugent scoring is ongoing with primary results available in early spring 2013.

Conclusion Immunologic mechanisms regulated by vitamin D may play a role in BV recurrence, but no previous study has examined whether supplementing women with vitamin D will impact subsequent development of BV. If effective against BV, vitamin D supplementation can have worldwide impact as a safe, simple intervention.