The causes of diarrhea after oral mesalazine in patients with ulcerative colitis may include irrational use of medication, individual differences, dietary irrationality, and exacerbation of the disease. 1. irrational use of drugs: ulcerative colitis patients oral mesalazine after diarrhea, may be due to the medication is not in accordance with the doctor’s instructions to take the reasonable cause, such as excessive dosage of medication, medication time irregularity, etc., can lead to the absorption of the drug is affected, resulting in diarrhea and other situations. 2. Individual differences: different individuals have different reactions to drugs, ulcerative colitis patients oral mesalazine diarrhea can not be ruled out due to individual differences. 3. Dietary irrationality: In addition to the above, during the medication, dietary intake is not reasonable, such as eating spicy stimulation, cold and other foods, may cause gastrointestinal dysfunction and thus diarrhea. 4. Disease aggravation: ulcerative colitis patients with aggravated diarrhea symptoms, can also lead to the occurrence of the above situation. Mesalazine should be used under the guidance of a medical professional, the discomfort is recommended to immediately consult a doctor, under the guidance of a medical professional to determine the cause of the disease, do not blindly use, in order to avoid adverse consequences.