Magnesium Side Effects: Diarrhea, Dose, and the 350 mg Limit
The most common side effect of magnesium pills is diarrhea, sometimes with nausea and stomach cramps. It depends on the dose and the form: keep supplements at or under 350 mg a day and choose a form that is not on NIH’s list of forms most linked to diarrhea, such as glycinate or citrate. Serious side effects are rare and mostly happen when the kidneys are weak.
| Side effect | How common | When it happens |
|---|---|---|
| Loose stools, diarrhea | Common: 11% to 37% of people on magnesium in cramp trials, vs 10% to 14% on placebo | Higher doses; oxide and other poorly absorbed forms |
| Nausea, stomach cramps | Common, often with diarrhea (NIH) | Same as diarrhea: higher doses and poorly absorbed forms |
| Too much magnesium in the blood | Rare in healthy people | Weak kidneys, or very large doses of magnesium laxatives or antacids |
| Medicines absorbed less | Predictable | Some antibiotics, osteoporosis drugs and thyroid pills taken at the same time |
Can magnesium cause diarrhea?
Yes, and it is the side effect most people get. Magnesium your gut does not absorb stays in the intestine, pulls in water and speeds things up (NIH). Magnesium laxatives, such as milk of magnesia and liquid magnesium citrate, are built on exactly this; their labels say they usually work in 30 minutes to 6 hours.
How often does it happen at supplement doses? The best count comes from a Cochrane review of 11 magnesium trials (735 people). Stomach and bowel effects, mostly diarrhea, hit 11% of people on magnesium in one trial (10% on placebo) and up to 37% in another (14% on placebo). Serious side effects were no more common than on placebo: 2 of 72 people on magnesium, 3 of 68 on placebo (Garrison 2020).
Some researchers think the 350 mg limit is too cautious. A 2023 review found that 7 of 10 studies, at 128 to 1,200 mg a day, saw no more diarrhea on magnesium than on placebo, and FDA’s adverse-event system held only 40 stomach cases tied to single-ingredient magnesium products (Costello 2023). Its authors run a group that promotes magnesium, so weigh it with that in mind.
Which forms cause the most diarrhea?
NIH names four forms as the ones most often reported to cause diarrhea: carbonate, chloride, gluconate and oxide. The less of a form you absorb, the more is left in the gut to pull in water. Oxide is the clearest case: only about 4% was absorbed in one study (Firoz 2001), and in a 60-day trial it raised urine magnesium no more than placebo, while citrate and an amino-acid chelate did (Walker 2003).
- Glycinate is not on NIH’s list. That is why it is often called gentle. In the one trial that compared glycinate with oxide at the same dose (100 mg, in 12 people who had part of their small bowel removed), overall absorption was about the same (23.5% vs 22.8%), and glycinate was better tolerated by every patient (Schuette 1994). One small study makes it a fair bet, not a proven fact.
- Citrate is not on NIH’s list either, and small studies found it absorbed better than oxide (NIH). But liquid citrate in large doses is sold as a laxative, so citrate pills can loosen stools as the dose climbs.
- The dose matters as much as the form. A review of magnesium absorption found the amount you take and your own magnesium level matter more than the form, and that several small doses are absorbed better than one large one (Schuchardt 2017).
Check the words in brackets on the magnesium line of the label. A “glycinate” powder or tablet can include oxide. All the forms compared.
How much magnesium is too much?
The upper limit for adults is 350 mg a day of magnesium from supplements and medicines. Food does not count toward it (NIH). The limit was set to avoid diarrhea, not organ damage: the Institute of Medicine chose it in 1997 with diarrhea as the deciding side effect (Costello 2023). The UK’s NHS says 400 mg or less a day of magnesium from supplements is unlikely to cause harm. NIH’s 350 mg limit is the stricter of the two.
| Age | Upper limit a day |
|---|---|
| 1–3 years | 65 mg |
| 4–8 years | 110 mg |
| 9 years and over, including pregnancy and breastfeeding | 350 mg |
Two things push people over the limit without meaning to:
- The label’s directions. We read 833 magnesium labels in NIH’s label database. The median serving was 250 mg, under the limit. But following the label’s own directions took the median day to 399 mg, over it. More from the label scan.
- Stacking products. Multivitamins, calcium-magnesium tablets, antacids and laxatives all add magnesium. So do hydrogen water tablets, at about 80 mg each. The “Magnesium” line on the Supplement Facts panel is the magnesium itself (the elemental amount), not the weight of the whole compound (NIH).
| Item | Value (mg) |
|---|---|
| Thorne glycinate, label maximum (4 capsules) | 480 mg |
| All labels: median day as directed | 399 mg |
| The upper limit for supplements | 350 mg |
| Nature Made citrate, 2 softgels | 250 mg |
| All labels: median serving | 250 mg |
| Nature Made glycinate, 2 capsules | 200 mg |
Signs of too much magnesium in the blood
Diarrhea is your body getting rid of extra magnesium. The serious problem is when it cannot: high blood magnesium (hypermagnesemia). Healthy kidneys can push out nearly all extra magnesium, so this mostly happens when kidney disease and a high intake come together (Ayuk 2014).
Warning signs (NIH): low blood pressure, nausea, vomiting, facial flushing, trouble passing urine, a blocked bowel, low mood and feeling very tired, then muscle weakness, trouble breathing, irregular heartbeat and cardiac arrest. Dangerous levels are rare. They have mostly come from very large doses of magnesium laxatives or antacids, usually more than 5,000 mg a day (NIH).
The numbers come from hospitals, not supplement users. Among 320 hospital patients given magnesium oxide as a laxative, 75 (23%) developed high blood magnesium. Risk went up with low kidney function (an eGFR at or below 55.4), doses of 1,650 mg of oxide a day or more, and 36 days or more of use (Wakai 2019). A Japanese case series described 4 people over 65 with weak kidneys who took magnesium oxide for constipation and could not easily report how they felt because of dementia or a stroke; one died (Yamaguchi 2019).
Weak kidneys do not always rule magnesium out. In a small 8-week trial in people with moderate to severe kidney disease, doctors gave magnesium under monitoring with no serious side effects from it (Bressendorff 2017). The difference is a doctor and a blood test.
Medicines magnesium affects, and medicines that lower magnesium
Magnesium can block these drugs (space them apart)
- Antibiotics: take tetracyclines (such as doxycycline) or quinolones (such as ciprofloxacin) at least 2 hours before or 4 to 6 hours after magnesium (NIH). In one study an antacid with magnesium and aluminum cut doxycycline levels to below the amount that works (Deppermann 1989); another found that moxifloxacin taken 2 hours before or 4 hours after the antacid lost less than a quarter of its absorption, which the authors judged not to matter (Stass 2001).
- Osteoporosis drugs (bisphosphonates such as alendronate): take them at least 2 hours apart from magnesium (NIH).
- Thyroid pills (levothyroxine). In a trial of 15 healthy adults, magnesium aspartate taken at the same time cut the thyroid hormone absorbed by 12%; citrate cut it by 7%, too small to be sure. The authors advise taking levothyroxine separately from magnesium (Attinger 2025).
- Zinc in very high doses. 142 mg of zinc a day lowered magnesium absorption in one study (NIH).
Not sure how many hours? Ask your pharmacist. How to time magnesium around medicines.
These drugs can lower your magnesium
- Heartburn pills (PPIs such as esomeprazole or lansoprazole), taken for a year or more, can lower magnesium. In about 1 in 4 of the cases FDA reviewed, supplements did not raise it and the PPI had to be stopped (NIH). A review of 9 studies (109,798 people) found PPI users were more likely to have low magnesium (Cheungpasitporn 2015).
- Water pills: loop and thiazide diuretics make you lose more magnesium; potassium-sparing ones make you lose less (NIH). In a Dutch study of 9,820 adults, only thiazide water pills were linked to low blood magnesium, mostly after long use; loop water pills were not (Kieboom 2018).
If you take one of these and have cramps, twitching or tiredness, your problem may be too little magnesium, not too much. Signs of low magnesium.
What to do if magnesium upsets your stomach
- Add it up. Find the magnesium line on each label, multiply by the servings you take, and add every product. Keep the total within the limit above.
- Take it with food, and split it. Take half with breakfast and half with dinner. Small doses are absorbed better (Schuchardt 2017).
- Take less. Adults who take no supplement get about 234 mg (women) to 268 mg (men) a day from food (NIH), so a pill of 100 to 200 mg often closes the gap.
- Change the form. Move from oxide (or a blend that includes oxide) to glycinate.
- Still loose? Stop and talk to your doctor. Diarrhea that does not settle has many causes.
Magnesium that is easier on the stomach
| Product | Magnesium | Why it is here | Cost | Buy |
|---|---|---|---|---|
| Revive MD Magnesium Glycinate | 100 mg a capsule | Glycinate, 200 mg in 2 capsules. On the Informed Sport list: every batch is tested for drugs banned in sport. It is not a check of the label. | $0.33 a day | Check price |
| Nature’s Truth Magnesium Glycinate | 100 mg a capsule | Glycinate, 200 mg in 2 capsules, with a meal. On NSF/ANSI 173, which checks the label. | $0.35 a day | Check price |
| Thorne Magnesium Glycinate | 120 mg a capsule | One capsule is a small dose to start with. On NSF/ANSI 173. | $0.29 a capsule | Check price |
| Nature Made Magnesium Citrate 250 mg | 125 mg a softgel | USP Verified, but citrate can loosen stools at higher doses. | $0.39 a day | Check price |
| KAL Magnesium Glycinate 350 | about 88 mg a capsule | 240 capsules, about 88 mg each. One serving (4 capsules, 350 mg) is the whole limit. Not on a testing list. | No list price | Check price |
Every magnesium product we track is ranked in the best magnesium guide.
Frequently Asked Questions
Can magnesium cause diarrhea?
Yes. Diarrhea is the most common side effect of magnesium pills, sometimes with nausea and stomach cramps (NIH). It comes from magnesium your gut does not absorb, which pulls water into the bowel. It is more likely above 350 mg a day from supplements and with forms such as oxide.
Why does magnesium cause diarrhea?
Magnesium that is not absorbed stays in the intestine and draws water in. It also speeds up the gut (NIH). Magnesium laxatives such as milk of magnesia work exactly this way.
Which form of magnesium is least likely to cause diarrhea?
NIH names carbonate, chloride, gluconate and oxide as the forms most often reported to cause diarrhea. Glycinate is not on that list, which is why it is often called gentle. In the one small trial that compared it with oxide at the same dose (12 people), glycinate was better tolerated by every patient (Schuette 1994). That is one small study, so treat “gentler” as likely, not proven.
Does magnesium glycinate cause diarrhea?
It can, at high doses, but it is not one of the forms NIH lists as most likely to. Keep the dose modest and split larger amounts between meals.
What are the side effects of taking too much magnesium?
In most people, diarrhea, nausea and cramps. Far more rarely, magnesium builds up in the blood. Early signs are low blood pressure, nausea, vomiting, facial flushing, trouble passing urine and feeling very tired; later ones are muscle weakness, trouble breathing, irregular heartbeat and cardiac arrest (NIH). Dangerous levels are rare. They have mostly come from very large doses of magnesium laxatives or antacids, usually more than 5,000 mg a day (NIH).
Does the 350 mg magnesium limit include food?
No. The upper limit for adults is 350 mg a day of magnesium from supplements and medicines. Food does not count toward it (NIH). Healthy kidneys clear the extra from food in urine (NIH).
Can magnesium supplements hurt your kidneys?
It is the other way round. If you have kidney disease, ask your doctor before you take magnesium. Weak kidneys cannot clear the extra, so it can build up (NIH). In a hospital study of 320 patients on magnesium oxide, 23% got high blood magnesium; low kidney function, high doses and long use raised the risk (Wakai 2019).
Does magnesium interact with medications?
Yes, both ways. It lowers how much of some antibiotics, osteoporosis drugs and thyroid pills you absorb, so space them. Heartburn pills (PPIs) and some water pills can lower your magnesium. The full list.
Related Guides
- Magnesium dosage guide: how much by age and sex
- Best time to take magnesium: food, splitting and medicines
- Glycinate vs citrate · all forms compared · gummies
- Signs of low magnesium · magnesium for seniors · magnesium guide
- Vitamin D side effects · melatonin side effects
Sources
- NIH Office of Dietary Supplements. Magnesium fact sheet for health professionals (updated January 6, 2026).
- Garrison SR, et al. Magnesium for skeletal muscle cramps. Cochrane Database Syst Rev. 2020. PubMed 32956536
- Costello R, et al. Perspective: call for re-evaluation of the tolerable upper intake level for magnesium supplementation in adults. Adv Nutr. 2023 (authors from the Center for Magnesium Education and Research). PubMed 37487817
- Firoz M, Graber M. Bioavailability of US commercial magnesium preparations. Magnes Res. 2001. PubMed 11794633
- Schuette SA, et al. Bioavailability of magnesium diglycinate vs magnesium oxide in patients with ileal resection. JPEN J Parenter Enteral Nutr. 1994. PubMed 7815675
- Walker AF, et al. Mg citrate found more bioavailable than other Mg preparations. Magnes Res. 2003. PubMed 14596323
- Schuchardt JP, Hahn A. Intestinal absorption and factors influencing bioavailability of magnesium: an update. Curr Nutr Food Sci. 2017. PubMed 29123461
- Ayuk J, Gittoes NJ. Contemporary view of the clinical relevance of magnesium homeostasis. Ann Clin Biochem. 2014. PubMed 24402002
- Wakai E, et al. Risk factors for the development of hypermagnesemia in patients prescribed magnesium oxide. J Pharm Health Care Sci. 2019. PubMed 30805197
- Yamaguchi H, et al. Severe hypermagnesemia induced by magnesium oxide ingestion: a case series. CEN Case Rep. 2019. PubMed 30136128
- Bressendorff I, et al. Oral magnesium supplementation in chronic kidney disease stages 3 and 4. Kidney Int Rep. 2017. PubMed 29142966
- Deppermann KM, et al. Influence of ranitidine, pirenzepine, and aluminum magnesium hydroxide on the bioavailability of various antibiotics. Antimicrob Agents Chemother. 1989. PubMed 2610502
- Stass H, et al. Influence of antacids and H2 antagonists on the absorption of moxifloxacin. Clin Pharmacokinet. 2001. PubMed 11352441
- Attinger MC, et al. Levothyroxine with magnesium citrate and magnesium aspartate: the ThyroMag trial. Clin Transl Sci. 2025. PubMed 41221788
- Cheungpasitporn W, et al. Proton pump inhibitors linked to hypomagnesemia: a systematic review and meta-analysis. Ren Fail. 2015. PubMed 26108134
- Kieboom BCT, et al. Thiazide but not loop diuretics is associated with hypomagnesaemia in the general population. Pharmacoepidemiol Drug Saf. 2018. PubMed 30095199
- Laxative onset: Milk of Magnesia and magnesium citrate oral solution drug labels, DailyMed. Labels: NIH Dietary Supplement Label Database (833 magnesium labels read August 29, 2026) and each brand’s page. Prices: each brand’s own list price, checked October 14, 2026.