Adjunct vaginal lactic acid with single-dose metronidazole for bacterial vaginosis: a randomized controlled trial

BACKGROUND: Bacterial vaginosis (BV) is a common cause of vaginal dysbiosis in women worldwide, often complicated by biofilm-associated recurrence. This randomized controlled trial aimed to evaluate whether adding a 7-day course of vaginal lactic acid to a single-dose oral metronidazole improves early ecological restoration and clinical outcomes compared with metronidazole alone. METHODS: An open-label, randomized controlled trial was conducted at the Department of Obstetrics and Gynaecology, Faculty of Medicine Siriraj Hospital, Mahidol University, Thailand, from December 2023 to January 2026. Women aged 18-50 years with symptomatic BV (Amsel criteria) were randomized (1:1) to receive oral metronidazole 2 g single-dose (MET) or metronidazole plus 7-day vaginal lactic acid 5 g daily (METLAC). The primary outcome was quantitative recovery of vaginal Lactobacillus at 10-14 days, assessed by 16S rRNA gene qPCR. Secondary outcomes included clinical cure, symptom resolution, and recurrence. RESULTS: A total of 110 women were enrolled and randomized. Three participants were lost to follow-up, leaving 107 for the clinical analysis (MET n = 53; METLAC n = 54). Primary qPCR analysis was available for 101 participants. At 10-14 days, Lactobacillus 16S rRNA gene copy numbers increased significantly from baseline in both groups (p < 0.01). While the between-group difference in absolute Lactobacillus recovery was not statistically significant, the METLAC group demonstrated a numerically greater mean recovery and a higher proportion of microbiological responders (51.9% vs. 44.9%). Secondarily, at one week, the participant-reported clinical cure rate was 100% (54/54) in the METLAC group compared with 75.5% (40/53) in the MET group (absolute difference 24.5%; p < 0.001). Resolution of malodorous discharge was also significantly faster in the METLAC group. Recurrence rates at six months were 31.4% in the METLAC group and 21.6% in the MET group. CONCLUSIONS: Adjunct vaginal lactic acid added to single-dose oral metronidazole did not significantly improve early vaginal Lactobacillus recovery or reduce 6-month bacterial vaginosis recurrence. However, women receiving adjunct therapy experienced a significantly higher one-week clinical cure rate and faster malodor relief, with only mild and tolerable vaginal irritation. These findings suggest that adjunctive vaginal lactic acid provides a feasible short-term symptomatic treatment option in resource-limited Southeast Asian settings, although its role in improving long-term microbiological outcomes remains uncertain. TRIALS REGISTRATION: Thai Clinical Trials Registry, TCTR20231106004 Registered on 6 November 2023.