Longitudinal vaginal microbiomes and quality-of-life patterns during tamoxifen therapy in breast cancer: a pilot study.

Tamoxifen is widely used in breast cancer treatment, but its effects on vaginal microbiome remain poorly understood. This prospective longitudinal pilot study explored vaginal microbiota profiles and quality-of-life parameters in women receiving tamoxifen for breast cancer in Seoul, South Korea (2023-2024). Eleven women initiating tamoxifen therapy were enrolled. Vaginal swabs were collected at baseline (V0) and 6 months (V6). Microbiota was profiled using 16 S rRNA gene sequencing. Quality of life was assessed using the 11-item Menopause Rating Scale. Participants were stratified by baseline colonization patterns. Overall community composition did not show a significant shift between baseline and 6 months. In the full-cohort taxa-level paired analysis, Gardnerella vaginalis (G. vaginalis) showed a nominal, non-FDR-significant increase from baseline to 6 months, and no taxon remained significant after multiple-comparison correction. A negative correlation was observed between G. vaginalis and Lactobacillus iners (L. iners) (ρ = -0.6, raw P < 0.01, FDR q < 0.05). Among participants with baseline G. vaginalis detection, 4 of 5 showed increased relative abundance at 6 months, although the confidence interval was wide. G. vaginalis abundance was associated with worse sexual-function-related quality-of-life scores in exploratory analyses, but item-level MRS comparisons did not remain significant after correction for multiple testing. In this small hypothesis-generating pilot cohort, women receiving tamoxifen showed largely stable overall vaginal community composition over 6 months, with an exploratory signal of G. vaginalis expansion among participants colonized at baseline. These findings should be interpreted cautiously given the small sample size, absence of a control group, treatment heterogeneity, and post hoc subgroup analysis, and require validation in larger controlled cohorts.