Darkening of the menstrual period after taking progesterone tablets may be caused by menstrual blood remaining in the vagina for too long. Progesterone tablets are commonly used in the treatment of irregular menstruation, the main purpose of which is medicated stripped bleeding. Menstruation is the cyclical shedding and bleeding of the endometrium that accompanies cyclic ovulation of the ovaries, and menstrual blood contains prostaglandins and a large amount of fibrinolytic enzymes from the endometrium. Menstruation sludges and oxidizes in the vagina and may manifest as darkened menstrual blood. Progesterone tablets belong to a class of progestogen drugs whose side effects include nausea, vomiting, migraine, bleeding, and impaired liver function. This drug is contraindicated in patients who are allergic to this drug, liver diseases, malignant tumors, and unexplained vaginal bleeding. It is recommended to take progesterone tablets under the guidance of a doctor, not blindly use the drug by yourself, so as not to bring adverse effects on the body. If the menstrual period is dark after taking progesterone tablets, it is recommended to go to the hospital for a clear examination, and then symptomatic treatment.