Adjunct Vaginal Lactic Acid With Metronidazole for Bacterial Vaginosis

Key Takeaways

• Lactobacillus recovery increased in both groups, without a statistically significant between-group difference at 10–14 days.
• Adjunct lactic acid was associated with higher one-week clinical cure and faster relief of malodorous discharge.
• Six-month recurrence was not lower with adjunct therapy, and adverse effects were limited to mild, tolerable vaginal irritation.

An open-label randomized controlled trial in Thailand found that adding 7 days of vaginal lactic acid to single-dose oral metronidazole improved one-week participant-reported cure to 100% versus 75.5% with metronidazole alone. In symptomatic women with bacterial vaginosis, oral metronidazole 2 g was compared with the same regimen plus vaginal lactic acid 5 g daily for 7 days. Early microbiologic recovery on the prespecified primary endpoint did not differ significantly between groups, leaving stronger short-term symptom improvement without a significant early microbiologic advantage.

The trial used an open-label randomized controlled design at the Department of Obstetrics and Gynaecology, Faculty of Medicine Siriraj Hospital, Mahidol University, Thailand. Enrollment ran from December 2023 through January 2026 and included women aged 18–50 years with symptomatic bacterial vaginosis defined by Amsel criteria. Participants were randomized 1:1 to oral metronidazole 2 g as a single dose alone or the same regimen plus vaginal lactic acid 5 g daily for 7 days. The primary endpoint was quantitative vaginal Lactobacillus recovery at 10–14 days by 16S rRNA gene qPCR, with 110 randomized, 3 lost to follow-up, 107 included clinically, and 101 included in the qPCR analysis. The comparison focused on early ecological recovery alongside short-term clinical response.

At 10–14 days, Lactobacillus 16S rRNA gene copy numbers increased significantly from baseline in both groups, with p values below 0.01. Absolute recovery did not differ significantly between groups, although the adjunct arm showed a numerically greater microbiologic response, including responder proportions of 51.9% and 44.9%, respectively. One-week participant-reported cure was markedly higher in the adjunct group, with an absolute difference of 24.5% and p<0.001, and malodorous discharge resolved faster. Early symptom improvement diverged from the nonsignificant primary microbiologic comparison.

Longer-term outcomes did not favor adjunct treatment, with six-month recurrence rates of 31.4% in the metronidazole-plus-lactic-acid group and 21.6% in the metronidazole-alone group. Adverse effects were limited to mild and tolerable vaginal irritation. The authors described adjunct vaginal lactic acid as a feasible short-term symptomatic option in resource-limited Southeast Asian settings, while noting uncertainty about long-term microbiologic benefit. 2023. Overall, the findings remained mixed between short-term symptom relief and longer-term recurrence.