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In patients with chronic idiopathic diarrhea resistant to standard treatment, opioids are often used as rescue therapy. This systematic review investigated opioid effects on gut function in chronic diarrhea. PubMed and Embase were searched regarding effects of opioid agonists on the gastrointestinal tract in humans with chronic or experimentally induced diarrhea. A total of 1472 relevant articles were identified and, after thorough evaluation, 11 clinical trials were included. Generally, studies reported a reduction in stool frequency and an increase in transit time during treatment with the opioid receptor agonists loperamide, asimadoline, casokefamide, and codeine compared with placebo. Loperamide and diphenoxylate significantly improved stool consistency compared with placebo, whereas asimadoline showed no such effects. Compared with placebo, loperamide treatment caused less abdominal pain and urgency. Asimadoline showed no significant subjective improvements, but fedotozine was superior to placebo in reducing abdominal pain and bloating in selected patients. Only two relevant studies were published within the last 20 years, and standardized endpoint measures are lacking. Most trials included few participants, and further evidence is needed from larger, prospective studies. Likewise, consensus is needed to standardize endpoints for stool frequency, transit time, and consistency to conduct future meta-analyses on opioids in management of chronic idiopathic diarrhea.