Opioids change gut motility, and opium tincture has been used for treatment of chronic diarrhoea for centuries. However, the effects have never been documented in controlled trials. We aimed to investigate the effects of opium tincture on gastrointestinal transit and motility, frequency of bowel movements, stool consistency, gastrointestinal symptoms and sedation. Twenty healthy subjects were included in this randomized controlled trial. Opium tincture or placebo was each applied for 9 days. Gastrointestinal transit and motility were investigated with the 3D-transit system. Bowel movements and gastrointestinal symptoms were recorded daily. General cognition, reaction time, memory and electroencephalography were used to assess effects on the central nervous system. Opium tincture doubled colonic transit (49 vs. 23 h, p < 0.001), decreased antegrade colonic movements (p < 0.05), reduced daily bowel movements (0.7 vs. 1.2, p < 0.001) and increased stool consistency (Type 3 vs. Type 4, p < 0.001). No changes in general cognition, reaction time or memory were observed, and minor changes of power observed by electroencephalography did not indicate sedation. This study is the first to show that opium tincture has anti-propulsive properties in the healthy gut, while no sedative effects were seen. This indicates that opium tincture is a relevant and safe treatment option in chronic diarrhoea.