Anirva sells Sleep Harmony, a dietary supplement that contains melatonin. This article shares research for general education and is not medical advice.
If your digestion has felt different since you started melatonin, it makes sense to wonder whether the two are connected. Melatonin may cause constipation or diarrhea in some people, but Mayo Clinic lists both as less common effects, and no one knows exactly how often it happens. In trials of melatonin for irritable bowel syndrome, how often people had bowel movements did not change. Other causes, such as added ingredients, medicines and routine changes, are also possible.
In this article
Key Takeaways
- Possible, but less common. Mayo Clinic lists loose stools, constipation, stomach cramps and nausea among less common melatonin effects.
- The rate is not well known. Trials rarely report these effects on their own. One 20 mg trial listed constipation in 4 of 39 people on melatonin and 1 of 38 on placebo, which is too few to be sure.
- Gut trials are reassuring, but limited. In trials of 3 to 6 mg in people with IBS, bowel movement frequency did not change.
- Other causes are common. Magnesium, 5-HTP, other medicines, diet changes and travel can all affect digestion.
- Melatonin is one part of Sleep Harmony. The studies looked at melatonin by itself, so they do not describe the whole formula.
What Do Medical Sources Say About Melatonin and Digestion?
Mayo Clinic lists headache, dizziness, nausea and daytime drowsiness as the most common melatonin effects. Its list of less common effects includes stomach cramps, nausea, loose stools and constipation.
A 2019 review of 50 controlled studies found melatonin's side effects were generally minor and short-lived. Its abstract named fatigue, mood and thinking or movement performance as the most commonly reported ones, and it did not single out digestive effects.
The gut is not a stranger to melatonin. The gut makes melatonin too, and researchers think it helps regulate gut movement, according to a 2023 trial report. That is why digestive effects are plausible, though plausible does not mean common.
What Have Trials Found About Melatonin and the Bowel?
Melatonin has been tested as a possible treatment for irritable bowel syndrome (IBS), and those trials also tell us about bowel effects. In a 2023 trial of 136 people with IBS, 6 mg a day for eight weeks improved pain, bloating and stool scores compared with placebo. How often people had bowel movements each week did not change. The authors said patients tolerated the supplement well and reported no specific side effects.
Two earlier trials of 3 mg at bedtime found no clear effect on stool type or how often people went. A 2013 trial of 3 mg plus 5 mg for six months in postmenopausal women found better symptoms in people with constipation-predominant IBS. The gain was not significant in people with diarrhea-predominant IBS. A 2022 review of four such trials, with 115 people, found better overall IBS severity and pain.
These trials involved people with IBS, and most were small, so they do not show what happens in healthy adults who take melatonin for sleep.
What About Higher Doses?
Higher doses may matter. High doses are more likely to cause side effects. In one trial, 39 women receiving radiotherapy for breast cancer took 20 mg of melatonin and 38 took placebo. The registered results list constipation in 4 of 39 on melatonin (10%) and 1 of 38 on placebo (3%), and abdominal pain in 12 of 39 (31%) and 8 of 38 (21%). These numbers are small, and radiotherapy can affect the gut, so they do not show that melatonin caused the difference.
What Else Can Cause Constipation or Diarrhea?
Other ingredients. Large doses of magnesium supplements can cause nausea, diarrhea and stomach cramps, and the suggested limit from supplements is 350 mg a day. 5-HTP's most common side effects include heartburn, stomach pain, nausea and diarrhea. For valerian, stomach upset is among the most common effects, and placebo groups reported it too. Probiotics usually have few side effects.
Medicines and supplements. NIDDK lists antacids with aluminum or calcium, iron supplements and some antidepressants among the things that can worsen constipation. Mayo Clinic lists antacids with magnesium and artificial sweeteners among causes of diarrhea.
Routine changes. NIDDK notes that travel, changes in what you eat, low fiber and low fluids can all change bowel habits. Mayo Clinic lists infections and food intolerances as causes of diarrhea. When melatonin starts at the same time as one of these, it can look like the cause by coincidence.
Melatonin Is One Part of Sleep Harmony
If you are taking Sleep Harmony, melatonin is one of several ingredients rather than the only one. Sleep Harmony combines melatonin, calming herbs and three probiotics in one formula, and we chose ingredients that have each been studied for relaxation and sleep.
The studies above looked at melatonin by itself, so they do not describe the whole formula, and the finished formula has not been clinically tested.
A Quick Look at the Research
| Question | Research | What It Found | How Solid | Worth Knowing |
|---|---|---|---|---|
| Digestive effects listed | Clinic side-effect lists | Loose stools, constipation, cramps, nausea: less common | Limited | No rate given |
| Bowel effects in IBS trials | One 136-person trial and smaller trials, 3–6 mg | No consistent change in how often people went | Limited | People with IBS, not healthy adults |
| High-dose digestive events | Registered results, 20 mg, 77 people | Constipation 4 vs 1; abdominal pain 12 vs 8 | Insufficient | Small; radiotherapy patients |
| Magnesium as a cause | University extension summary | Large doses can cause diarrhea | Moderate | Depends on amount and form |
| 5-HTP as a cause | NIH MedlinePlus summary | Diarrhea and stomach pain are common | Moderate | Summary, not a single trial |
Good to Know
How to Tell if Melatonin Is the Cause
Timing is the best clue. It can help to ask whether the change began after you started melatonin, and whether it eases when you stop. Changing one thing at a time makes it easier to tell, and it helps to note your dose, other supplements, medicines and any changes in food, travel or stress.
If symptoms are severe, keep going, or come with blood in the stool or a fever, a healthcare provider can help.
The Short Version
Can melatonin cause constipation or diarrhea? It may in some people, but it is a less common effect, and the rate is not well measured. In trials of people with IBS, melatonin did not change how often they went. Dose, other ingredients, medicines and routine are all worth a look before pointing to melatonin.
Frequently Asked Questions
Can melatonin cause constipation?
Can melatonin cause diarrhea, and does it happen often?
Can higher doses of melatonin cause stomach problems?
Which sleep supplement ingredients can affect digestion?
What should I do if melatonin upsets my stomach?
Does Sleep Harmony work the same way as melatonin alone?
References
- Mayo Clinic Staff. Melatonin side effects: What are the risks? Mayo Clinic. mayoclinic.org
- Foley HM, Steel AE. Adverse events associated with oral administration of melatonin: a critical systematic review of clinical evidence. Complement Ther Med. 2019;42:65–81. pubmed.ncbi.nlm.nih.gov PMID 30670284.
- Dinevari MF, Jafarzadeh F, Jabbaripour Sarmadian A, et al. The effect of melatonin on irritable bowel syndrome patients with and without sleep disorders: a randomized double-blinded placebo-controlled trial. BMC Gastroenterol. 2023;23:135. link.springer.com DOI 10.1186/s12876-023-02760-0.
- ClinicalTrials.gov. Melatonin supplementation for cancer-related fatigue in patients receiving radiotherapy (NCT02332928): adverse events table. clinicaltrials.gov NCT02332928.
- Mukhopadhyay ND, Khorasanchi A, Pandey S, et al. Melatonin supplementation for cancer-related fatigue in patients with early stage breast cancer receiving radiotherapy: a double-blind placebo-controlled trial. Oncologist. 2024;29(2):e206–e212. pubmed.ncbi.nlm.nih.gov PMID 37699115; DOI 10.1093/oncolo/oyad250.
- AARP. Can melatonin help you sleep? And is it safe? November 2025. aarp.org
- University of Florida IFAS Extension. Magnesium (fact sheet FY1088). edis.ifas.ufl.edu
- MedlinePlus. 5-HTP. National Library of Medicine. medlineplus.gov
- NIH Office of Dietary Supplements. Valerian: fact sheet for health professionals. ods.od.nih.gov
- National Center for Complementary and Integrative Health. 5 things to know about probiotics. NIH. nccih.nih.gov
- NIDDK. Symptoms and causes of constipation. NIH. niddk.nih.gov
- Mayo Clinic. Diarrhea: causes. Mayo Clinic. mayoclinic.org
- National Center for Complementary and Integrative Health. Melatonin: what you need to know. NIH. nccih.nih.gov
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