BACKGROUND: Over-the-counter intra-vaginal lactic-acid containing douches are marketed as vaginal hygiene products that support optimal vaginal pH balance. We report the effect of a commercially available douche (Etos®) on the vaginal microbiota (VM) in a prospective study. RESULTS: Twenty-five healthy women were recruited through advertisements in 2015-2017 (ethical approval: METC-2014_413) and followed over three menstrual cycles. The participants had a median age of 24 years [IQR: 22-29], were mostly Dutch-Caucasian (88%), and 60% used combined oral contraceptives. All participants douched three times a week during the second cycle, starting on the first day of that cycle. Participants completed a questionnaire at baseline, kept a daily diary to report douching, menses, and sexual activity, self-collected vaginal swabs every other day during the first and third cycle and daily during the second cycle, and measured vaginal pH mid-cycle. A median of 44 vaginal swabs [inter-quartile range (IQR): 41-50] were assessed per participant by 16S rRNA gene (V3-V4 region) sequencing and a Candida albicans PCR was done at four time-points. At baseline, 21 participants (84%) had Lactobacillus-dominated VM (Lactobacillus crispatus (n = 14), L. iners (n = 6), or diverse Lactobacillus species (n = 1) and 4 participants (16%) had VM consisting of diverse anaerobes. In multinomial logistic regression models, a trend towards increased odds were observed for having diverse anaerobic VM in the second and third cycle, compared to the first cycle, after adjusting for menses [odds ratio (OR) = 1.4 (95% CI: 0.9-2.1) and OR = 1.7 (95% CI: 0.9-3.1), respectively] (p = 0.376). Douching did not affect vaginal pH. Menses increased the odds for having VM consisting of diverse anaerobes almost two-fold (OR = 1.7; 95% CI: 1.0-2.8), while douching during menses increased the odds 2.6 fold (OR = 2.6; 95% CI: 1.0-6.5), compared to not menstruating (p = 0.099). Participants were more likely to test positive for C. albicans after cycle 2, compared to cycle 1 [OR = 3.0 (95% CI: 1.2-7.2); p = 0.017]. CONCLUSION: The Etos® douche did not significantly affect the vaginal pH or VM composition, although increased odds for having diverse anaerobic VM was observed, especially when douching during menses. Furthermore, douching may promote C. albicans infections.