After taking lactulose, patients usually start to have a bowel movement after 8-12 hours. However, the time of onset of action varies depending on the individual’s body type, type of disease, and other factors. In clinical practice, lactulose is a common drug used to treat chronic constipation and is also used to treat or prevent hepatic encephalopathy caused by liver failure. It can be broken down into weak acids in the colon, which in turn draws water into the colon, thus helping to soften dry stools and promote the entry of metabolites into the excretory system, achieving therapeutic relief of constipation. However, depending on the severity of the patient’s constipation, the onset of action may vary. In most cases, if the patient has mild constipation, the effect can usually be seen within a few hours (mostly around 8-12 hours), but if the constipation is severe, it may take 1-2 days to see the effect. It should be noted that due to the fructose content in lactulose, it should be used with caution for patients with underlying conditions such as hypertension, diabetes and coronary heart disease.