Lactose intolerance due to diarrhea is secondary and recovery time varies depending on the individual’s condition, usually within 2-8 weeks. Bacterial, viral or parasitic intestinal infections can lead to chronic diarrhea, causing extensive damage to the intestinal epithelial cells and affecting the villi of the intestinal mucosa, which are not able to secrete lactase enzyme well, so that part of the lactose can not be broken down and absorbed, resulting in secondary lactose intolerance. The damaged villi of the intestinal epithelium usually need to be repaired in 2-8 weeks, when the lactase secretion gradually returns to normal. Depending on the individual’s physical condition, it may take at least 2 weeks for fast recovering patients and up to 2 months for slow recovering patients. If diarrhea causes lactose intolerance, it is recommended to go to the hospital as soon as possible, under the guidance of the doctor for targeted treatment.