Quick answer
NIH says high doses of magnesium from supplements or medicines can cause diarrhea, nausea and abdominal cramping. In trials pooled by a Cochrane review, gut side effects affected 11% to 37% of people taking oral magnesium and 10% to 14% of those on placebo. Ask your doctor first if you have kidney disease.1,2
Key facts
- NIH says high intakes of magnesium from supplements and medicines can cause diarrhea, nausea and abdominal cramping.1,3
- The forms most often reported to cause diarrhea are magnesium carbonate, chloride, gluconate and oxide.3
- NIH says too much magnesium from food does not pose a health risk in healthy people, because the kidneys eliminate the excess.3
- In the pooled trials, mostly gut side effects occurred in 11% to 37% of oral magnesium users and 10% to 14% of placebo users.2
- A magnesium citrate laxative is labeled to produce a bowel movement generally within half an hour to 6 hours.4
What NIH says about magnesium and diarrhea
High intakes of magnesium from supplements and medicines can cause diarrhea, nausea and abdominal cramping.1
The laxative effect comes from unabsorbed magnesium salts drawing water into the intestine and colon, and from faster movement of the gut.3
NIH adds that the problem comes from supplements and medicines, not from food. Magnesium that occurs naturally in food is cleared by the kidneys in healthy people, and the upper limit of 350 mg a day for adults counts only magnesium from supplements and medicines.1,3
Which forms of magnesium are most often named
The forms most often reported to cause diarrhea are magnesium carbonate, chloride, gluconate and oxide.3
Magnesium oxide, one of those forms, is sold as an antacid, and the label on such tablets says the product may have a laxative effect. Magnesium citrate is not on NIH's list, but it is sold as a saline laxative, and its label says it generally produces a bowel movement in half an hour to 6 hours.4,5
The sources checked do not include a head-to-head trial comparing glycinate with citrate for diarrhea, so they cannot rank forms. They do not address magnesium malate or threonate either.3
What trials found about how often it happens
In the trials pooled in a Cochrane review, mostly gut side effects such as diarrhea affected 11% to 37% of people taking oral magnesium, against 10% to 14% of those on placebo.2
That review pooled 11 trials in 735 people taking magnesium for muscle cramps, including older adults and pregnant women, and it counted minor side effects as a group. The forms and doses varied, so the range of 11% to 37% is a rough guide, not a rate for any one product.2
In the four-week trial of 250 mg of magnesium a day as bisglycinate, side effects were no more common than with placebo, and 93% of participants reported none.6
Who should ask their doctor
Diarrhea from a supplement is usually a dose and form question, but a few situations call for a professional. Ask your doctor or pharmacist before using magnesium if any of these apply.3
- You have kidney disease. NIH says the risk of toxicity rises when the kidneys cannot remove extra magnesium.3
- You already use a magnesium laxative or antacid. Some products carry large amounts that count toward the limit.3,4
- You have abdominal pain, nausea or vomiting, or a sudden change in bowel habits lasting 2 weeks. The magnesium citrate laxative label lists these as reasons to ask a doctor before use.4
- You take medicines such as tetracycline or quinolone antibiotics, oral bisphosphonates or diuretics. NIH names them as medicines magnesium may interact with.3
Questions people ask
Why does magnesium cause diarrhea?
NIH says unabsorbed magnesium salts in the intestine and colon draw water in and speed the movement of the gut. This is the same action that makes some magnesium salts work as laxatives.3
Which magnesium causes the least diarrhea?
The sources checked do not rank forms. NIH names carbonate, chloride, gluconate and oxide as the forms most often reported to cause diarrhea, and one four-week trial of 250 mg a day of magnesium bisglycinate found no more side effects than placebo.3,6
Does magnesium cause bloating or gas?
The NIH sheets name diarrhea, nausea and abdominal cramping, and do not list bloating or gas. The sources checked do not address them, which is not a finding either way.1,3
Does magnesium glycinate cause diarrhea?
NIH does not name glycinate among the forms most often reported to cause diarrhea, but that list is not a head-to-head comparison. Ask your pharmacist how your form and amount fit with your other products.3
Can too much magnesium be dangerous?
Very large doses from laxatives and antacids, typically above 5,000 mg a day, have caused magnesium toxicity, including a few deaths, and the risk rises with poor kidney function. Ask your doctor if you have kidney disease.3
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Sources
- Office of Dietary Supplements, National Institutes of Health. Magnesium: Fact Sheet for Consumers. Updated March 22, 2021. Opened 2026-10-06.
- Garrison SR, et al. Magnesium for skeletal muscle cramps. Cochrane Database Syst Rev. 2020;9(9):CD009402. Opened 2026-10-01.
- National Institutes of Health, Office of Dietary Supplements. Magnesium: Fact Sheet for Health Professionals. Updated January 6, 2026. Opened 2026-10-06.
- Walgreens Magnesium Citrate Oral Solution, saline laxative: Drug Facts label (Walgreen Co.). DailyMed, U.S. National Library of Medicine. Label effective July 20, 2026. Opened 2026-10-07.
- Magnesium Oxide 400 mg tablets, antacid: Drug Facts label (Westminster Pharmaceuticals). DailyMed, U.S. National Library of Medicine. Label effective July 6, 2026. Opened 2026-10-07.
- Schuster J, Cycelskij I, Lopresti A, Hahn A. Magnesium Bisglycinate Supplementation in Healthy Adults Reporting Poor Sleep: A Randomized, Placebo-Controlled Trial. Nat Sci Sleep. 2025;17:2027-2040. Opened 2026-10-01.
This page is information, not medical advice. Talk to your doctor or pharmacist about your own situation.