Melatonin

Melatonin is the hormone the pineal gland releases at night, but far more of it is produced in the gut, where it is involved in regulating motility and sensation PMID 15914575. That dual role led a Singapore group to test it in people with IBS, on the theory that it might help bowel symptoms and sleep at the same time. In the United States it is sold as a dietary supplement; in some other countries it is a prescription medicine.

The trials are small and old but consistently designed around 3 mg at bedtime. The best known randomised 40 patients with IBS and sleep disturbance to melatonin or placebo for two weeks, with rectal manometry and overnight polysomnography before and after. Melatonin reduced the mean abdominal pain score by 2.35 points versus 0.70 on placebo and raised the rectal pain threshold, but bloating, stool type, stool frequency, anxiety, and depression did not change, and, strikingly, neither did any objective sleep parameter PMID 15914575. The same group then ran an eight-week crossover trial in 17 women: IBS scores improved by 3.9 points on melatonin versus 1.3 on placebo, 88% versus 47% reported at least mild improvement, and again sleep, anxiety, and depression scores were unchanged PMID 16268966. An Indian pilot of 18 patients over eight weeks reported a 45% improvement in overall IBS score with melatonin versus 16.7% with placebo, with gains in extracolonic symptoms and quality of life PMID 17198061.

A 2023 meta-analysis pooled four randomised trials with 115 participants in total and found greater improvement in overall IBS severity with melatonin than placebo (Hedges’ g 0.746, confidence interval 0.401 to 1.091), along with better pain scores and quality of life, but no significant effect on abdominal distension or on sleep quality; safety was similar to placebo PMID 36257872. The authors registered the review prospectively, but 115 people across four studies is a thin base for an effect size of that magnitude.

Points clinicians commonly raise: the effect, if real, appears to run through gut sensation rather than sleep, so it is not simply a matter of sleeping better; every trial used 3 mg at night for two to eight weeks, and nothing is known about higher doses or longer use in IBS; melatonin can cause next-day drowsiness and interacts with some medicines, including anticoagulants and sedatives; and supplement products vary in actual content, which matters when the tested dose is a single, specific one.

Talk to a clinician before starting or stopping anything.