Active ingredient: Metronidazole

There is no evidence that metronidazole impairs female fertility or directly affects the hypothalamic-pituitary-ovarian axis, ovulation, follicular development, luteal-phase duration, or menstrual cycle length. Metronidazole is not expected to affect the progesterone-related post-ovulatory temperature rise. However, fever, systemic illness, gastrointestinal symptoms, or sleep disruption during treatment may temporarily reduce the reliability of BBT observations. Metronidazole has no established direct effect on cervical mucus production. However, when used to treat bacterial vaginosis (BV), it reduces BV-associated anaerobic bacteria and alters the vaginal microbial environment. Changes in vaginal discharge during treatment may therefore affect the woman’s perception or recording of cervical mucus, even without any change in ovulation or ovarian function. BV itself is characterized by disruption of the Lactobacillus-dominant vaginal microbiome and increased abundance of anaerobic bacteria. Metronidazole can reduce these BV-associated bacteria and may support re-establishment of Lactobacillus colonization. Dry mouth and gastrointestinal adverse effects may occur during treatment. If vomiting or diarrhoea results in true dehydration, reduced hydration could theoretically affect the volume or consistency of secretions; however, this would be a nonspecific indirect effect rather than a direct pharmacological effect of metronidazole. Key consideration: When interpreting changes in cervical mucus or other fertility biomarkers, the underlying infection, particularly BV, should be considered as an important confounding factor.