Bacterial vaginosis (BV) develops when the concentration of healthy Lactobacillus species in the vagina declines and is replaced by other bacterial species. BV is the most common vaginal infection worldwide, but the etiology of this complex condition is not clear. BV is associated with a 60% increased risk of HIV acquisition as well as numerous other detrimental reproductive outcomes. A profound racial disparity exists in BV prevalence in women in the United States (US): 23% of white women versus. 52% of black women have BV. The investigators hypothesize that inadequate vitamin D contributes to BV development and/or recurrence. Vitamin D is essential to immune function, serving both to stimulate mechanisms associated with pathogen elimination and to regulate immune response. According to nationally-representative data, 90% of US blacks have insufficient vitamin D levels. In two recent analyses, low vitamin D was associated with higher BV prevalence in pregnant African-Americans; a third replicated this finding in pregnant African-American and white women. The investigators wish to conduct a small, pilot randomized controlled trial (RCT) to assess the effect of vitamin D supplementation among non-pregnant, BV-positive women at a public sexually transmitted disease (STD) clinic. This small (n=150), two-arm, placebo-controlled, masked, 24-week RCT of high-dose vitamin D supplementation will inform the development of future large-scale RCT design and implementation.

Inclusion Criteria:

• speak English;
• be between 18 and 50 years old, inclusive;
• be pre-menopausal;
• have at least one ovary;
• be positive for bacterial vaginosis

Exclusion Criteria:

• pregnant at enrollment or in the previous 3 months;
• planning to become pregnant in the next six months;
• currently breastfeeding;
• currently be menstruating heavily;
• have a contraindication to oral metronidazole treatment