Generally speaking, oral metronidazole does not affect menstruation, but topical metronidazole is prohibited during menstruation. Metronidazole is widely used in the treatment of anaerobic infections and trichomonas vaginitis in women, and is an antibiotic. Adverse gastrointestinal reactions such as nausea, vomiting and abdominal pain may occur when taken orally, but it does not affect menstruation. Menstruation is formed by the cyclical shedding of the uterine lining stimulated by estrogen and progesterone, and the use of antibiotics does not block this process, so oral metronidazole does not affect menstruation. It should be noted that metronidazole can be taken orally during menstruation but cannot be used externally, so as not to cause an upward infection. Metronidazole is a prescription drug, and it is recommended that you follow your doctor’s instructions.