You should not smoke before having a colonoscopy. Colonoscopy is the main test used to diagnose intestinal diseases. Fasting is required on the day of the colonoscopy, and bowel preparation is needed before the examination, such as oral laxatives such as compound polyethylene glycol electrolyte dispersions and lactulose oral solution as prescribed by the doctor. These drugs can promote the contraction of intestinal smooth muscle and increase the secretion of fluid in the intestinal lumen to achieve the effect of emptying the intestine. The process of oral medication to clean the intestines may cause some burden and irritation to the gastrointestinal tract, while cigarettes contain nicotine and tobacco oil, which can also damage the mucous membrane of the gastrointestinal tract and cause gastrointestinal discomforts and other manifestations, which can have a direct impact on physical health. In addition, smoking can affect lung function and induce lung disease. If painless colonoscopy also requires sedation, anesthesia can affect breathing, so don’t smoke before doing colonoscopy.