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.