Does magnesium help with constipation?

Magnesium oxide does, in the trials that tested it. In a meta-analysis of 2 randomized trials in adults with chronic constipation, 68% responded to magnesium oxide against 19% on control, with 3.72 more bowel movements a week. Both trials were in Japan, at 1.5 g a day for four weeks, in about 94 people, most of them women. No other magnesium supplement has been tested this way in adults with chronic constipation. The other trials on this page used magnesium-rich mineral water or were in children.

Unsettled. Two expert bodies read the same two trials and came to different strengths. The British Dietetic Association strongly recommends magnesium oxide on what it calls moderate evidence. The American gastroenterology societies make only a conditional recommendation, because their own grading put the certainty at very low. Two small trials from one country is the reason the answer is yes and the confidence is limited.

What the trials found

The clearest single trial ran for 28 days in 90 adults with chronic idiopathic constipation, mean age 42 and 93% women. Overall improvement came in 68.3% on 1.5 g of magnesium oxide a day, 69.2% on senna and 11.7% on placebo. All 90 finished the study, and none had a severe treatment-related side effect. That trial ran in Japan and is one of the two in every pooled analysis on this page.

The American Gastroenterological Association and American College of Gastroenterology pooled the same two trials for their 2023 guideline. Magnesium oxide added about 3.59 spontaneous bowel movements a week, from 2.64 to 4.54, and made a response 3.93 times as likely. Diarrhea was not detectably more common than on placebo, a relative risk of 1.07 with an interval from 0.65 to 1.74. They graded the overall certainty of this evidence as very low.

Mineral water rich in magnesium

Across trials with 539 people, high-mineral water made a response 1.47 times as likely as low-mineral water. These waters carry sulfate along with magnesium, and the analysis cannot separate which of the two did the work. In a 14-day trial of 226 outpatients, 1.1 quarts (1 liter) a day of a magnesium and sulfate rich water gave a response in 50% against 29% on low-mineral water, and the water's manufacturer funded it. In a 6-week trial of 106 otherwise healthy adults, 17 fl oz (500 mL) a day of a magnesium sulfate water improved stool consistency. Its main outcome, complete bowel movements, was only a trend on a one-sided test.

Children

In a 1-month trial of 100 children aged 2 to 12 with spastic cerebral palsy and chronic constipation, oral magnesium sulfate worked in 68% against 9.5% on placebo. That is a specific group, and magnesium laxatives for children are a decision for their doctor.

Unsettled. A citrate or glycinate capsule is not proven to do what the oxide did. For adults with chronic constipation, the AGA-ACG guideline says only magnesium oxide has been tested in randomized trials for this, and the effect of citrate, glycinate, lactate, malate or sulfate is unknown. A narrative review notes that citrate dissolves readily and may move the bowels more than oxide, which is a reason to expect different behavior and no evidence of the size.

Who should be careful

The laxative effect is the point of the dose here, and the same effect is the commonest side effect for anyone else. High doses of magnesium from supplements or medicines often cause diarrhea with nausea and abdominal cramping. The US National Institutes of Health sets the upper limit for magnesium from supplements and medicines at 350 mg a day for adults, and lower for children, from 65 to 350 mg depending on age. The 1.5 g of magnesium oxide used in the trials sits above that, so it is a treatment dose, to take only on medical advice.

Not for everyone. Reduced kidney function is the real danger. The kidneys clear excess magnesium, so the risk of toxicity rises as they fail, and the AGA-ACG guideline says magnesium supplements should be avoided below a creatinine clearance of 20. Among older patients on magnesium oxide at one Japanese hospital, high blood magnesium reached 53.1% in those with both low kidney clearance and doses of 900 mg a day or more, a threshold the 1.5 g trial dose is above. That is observational data from one center.

Two case reports describe severe high blood magnesium in people taking magnesium hydroxide for constipation, and one of them died. One was an 82-year-old woman with end-stage kidney disease. The other was a 50-year-old woman with normal kidney function. Case reports cannot say how often this happens, and they do show it is possible with normal kidneys.

If you take levodopa for Parkinson's disease, magnesium oxide lowered blood levels of levodopa by 35.3% and of carbidopa by 80.9% in a study of healthy volunteers. That study measured drug levels, not symptoms, and it was not randomized. It is enough to raise with the doctor who prescribes the levodopa.

What expert bodies say

The AGA and ACG give magnesium oxide a conditional recommendation for chronic idiopathic constipation and note that the doses in the trials were higher than those used in practice. The British Dietetic Association gives it a strong recommendation, saying it increases the number of people with constipation who get a clinical benefit. Both rest on the same two trials.

How we searched

What would change this answer

• Randomized trials outside Japan, at the lower doses people actually take, around 0.5 to 1 g a day.
• Any randomized trial of magnesium citrate, glycinate or magnesium from food for constipation. We found none.
• Trials longer than four weeks, with blood magnesium measured in older people. Every magnesium oxide trial we found lasted a month or less.

For magnesium in food, how much you need and who runs short, see the magnesium guide.

Sources

1. ev-mgcon-01 · Meta-analysis or systematic review · systematic review and meta-analysis of RCTs · n = —
   Overall, 68% responded to magnesium oxide and 19% to control (RR 3.32 [1.59, 6.92]; p = 0.001). Magnesium oxide improved stool frequency (MD 3.72 bowel movements/week [1.41, 6.03]; p = 0.002) and consistency (MD 1.14 Bristol points [0.48, 1.79]; p = 0.0007).
   
   Who: Adults with chronic constipation, 2 RCTs of magnesium oxide within an 8-RCT review
   Effect: Response RR 3.32 (95% CI 1.59 to 6.92); stool frequency MD 3.72 per week (1.41 to 6.03); Bristol scale MD 1.14 (0.48 to 1.79)
   Certainty: n=787 is the whole review of 8 RCTs; magnesium accounts for 2 RCTs (about 94 people, per BDA). Both in Japan, 1.5 g/day, 4 weeks.
   Neurogastroenterol Motil, 2023 · checked 2026-10-06 · we read the abstract
2. ev-mgcon-02 · Meta-analysis or systematic review · guideline systematic review with meta-analysis of RCTs · n = 94
   Compared with placebo, treatment with MgO may increase the number of CSBMs per week (MD 4.29, 95% CI 2.93–5.65) and SBMs per week (MD 3.59, 95% CI 2.64–4.54). Participants treated with MgO achieved a higher treatment response compared with placebo (RR 3.93, 95% CI 2.04–7.56).
   
   Who: 94 adults with chronic idiopathic constipation, 93% women, 1.5 g/day magnesium oxide for 4 weeks
   Effect: SBM MD 3.59 per week (2.64 to 4.54); response RR 3.93 (2.04 to 7.56); diarrhea RR 1.07 (0.65 to 1.74)
   Certainty: Overall certainty of evidence for MgO by GRADE: very low; only 2 Japanese RCTs.
   Gastroenterology, 2023 · checked 2026-10-06 · we read the fulltext
3. ev-mgcon-03 · Meta-analysis or systematic review · systematic review and meta-analysis of RCTs · n = 539
   High-mineral water resulted in higher response to treatment than low-mineral water (RR: 1.47, [1.20-1.81], n = 539).
   
   Who: Adults with chronic constipation in RCTs of high-mineral water
   Effect: Response RR 1.47 (95% CI 1.20 to 1.81)
   Certainty: Waters with magnesium and sulfate together; the contribution of magnesium specifically is not separated.
   Aliment Pharmacol Ther, 2024 · checked 2026-10-06 · we read the abstract
4. ev-mgcon-04 · Randomized controlled trial(s) · double-blind randomized placebo-controlled trial · n = 90
   The response rate for overall improvement was 11.7% in the placebo group, 69.2% in the senna group, and 68.3% in the MgO group (P < 0.0001).
   
   Who: 90 patients with chronic idiopathic constipation, mean age 42, 93% women, 28 days
   Effect: Overall improvement 68.3% MgO, 69.2% senna, 11.7% placebo (P < 0.0001)
   Certainty: Japan; one of two RCTs included in the meta-analyses above.
   Am J Gastroenterol, 2021 · checked 2026-10-06 · we read the abstract
5. ev-mgcon-05 · Randomized controlled trial(s) · multicentre double-blind randomized controlled trial · n = 226
   Treatment response was more frequent in the Hépar arm than in the control group at day 14 (50% versus 29%, respectively; P = 0.001).
   
   Who: 226 outpatients with functional constipation, 1 L/day for 14 days
   Effect: Response at day 14: 50% versus 29% (P = 0.001)
   Certainty: Hépar water: magnesium together with sulfate; funded by the manufacturer.
   Nutrition, 2019 · checked 2026-10-06 · we read the abstract
6. ev-mgcon-06 · Randomized controlled trial(s) · double-blind randomized placebo-controlled trial · n = 106
   The daily consumption of a natural mineral water rich in magnesium sulphate and sodium sulphate improved bowel movement frequency and stool consistency in subjects with functional constipation.
   
   Who: 106 otherwise healthy subjects with functional constipation; 75 in the 17 fl oz (500 mL) arms; 6 weeks
   Effect: Stool consistency improved (p < 0.001); complete spontaneous bowel movements tended higher (one-sided p = 0.036)
   Certainty: The primary endpoint was only a trend by a one-sided test; the 10 fl oz (300 mL) arms were stopped.
   Eur J Nutr, 2017 · checked 2026-10-06 · we read the abstract
7. ev-mgcon-07 · Randomized controlled trial(s) · double-blind randomized placebo-controlled trial · n = 100
   Effective safe treatment was achieved in 31 (68%) and 4 (9.5%) patients in the O-Mg and placebo groups, respectively (RR, 2.95; 95% CI 2.0-4.5) (P < 0.001).
   
   Who: 100 children aged 2 to 12 with spastic cerebral palsy and chronic constipation, 1 month
   Effect: Effective treatment 68% versus 9.5% (RR 2.95, 95% CI 2.0 to 4.5)
   Certainty: Specific group; magnesium laxatives for children only as prescribed by a physician.
   World J Pediatr, 2021 · checked 2026-10-06 · we read the abstract
8. ev-mgcon-08 · Position of an expert body · clinical practice guideline · n = —
   Only MgO has been evaluated in RCTs; the bioavailability and clinical efficacy of other formulations of magnesium (eg, citrate, glycinate, lactate, malate, sulfate) for CIC are unknown.
   
   Who: Adults with chronic idiopathic constipation
   Effect: Conditional recommendation; only MgO studied in RCTs
   Certainty: Conditional recommendation due to very low certainty; doses in RCTs are higher than in practice.
   Gastroenterology, 2023 · checked 2026-10-06 · we read the fulltext
9. ev-mgcon-09 · Position of an expert body · clinical practice guideline · n = —
   Magnesium oxide supplements increase the number of people with constipation who have a clinical benefit. Level of evidence: moderate Strength of recommendation: strong
   
   Who: Adults with chronic constipation
   Effect: Strong recommendation, moderate level of evidence
   Certainty: Discrepancy between authorities: BDA strong, AGA-ACG conditional, on the same two RCTs.
   Neurogastroenterol Motil, 2025 · checked 2026-10-06 · we read the fulltext
10. ev-mgcon-10 · Position of an expert body · government fact sheet · n = —
    The Tolerable Upper Intake Level for supplemental magnesium is 350 mg for adults, and it ranges from 65 to 350 mg for children and adolescents, depending on age.
    
    Who: Adults; children 65 to 350 mg depending on age
    Effect: UL for supplemental magnesium 350 mg/day (adults)
    Certainty: UL is about side effects in healthy people; therapeutic doses of MgO (1.5 g) exceed the UL as prescribed.
    NIH Office of Dietary Supplements, Magnesium Fact Sheet for Health Professionals · checked 2026-10-06 · we read the section
11. ev-mgcon-11 · Position of an expert body · government fact sheet · n = —
    The risk of magnesium toxicity increases with impaired renal function or kidney failure because the ability to remove excess magnesium is reduced or lost
    
    Who: People with impaired renal function or kidney failure
    Effect: Higher risk of magnesium toxicity
    Certainty: A general warning from the agency.
    NIH Office of Dietary Supplements, Magnesium Fact Sheet for Health Professionals · checked 2026-10-06 · we read the section
12. ev-mgcon-12 · Position of an expert body · clinical practice guideline · n = —
    hypermagnesemia is more likely to occur in individuals with significant renal impairment and magnesium supplements should be avoided in those with a creatinine clearance of <20 mg/dL
    
    Who: People with significant renal impairment
    Effect: Avoid magnesium supplements below creatinine clearance 20
    Certainty: The unit in the guideline text is given as mg/dL (as in the source).
    Gastroenterology, 2023 · checked 2026-10-06 · we read the fulltext
13. ev-mgcon-13 · Observational data · case-control study · n = 175
    the incidence of hypermagnesemia was shown to increase to 11.6% for those with MgO dose ≥ 900 mg/day, 27.0% for those with eCcr ≤ 28.2 mL/min, and 53.1% for those with both.
    
    Who: Older patients prescribed MgO, 35 cases and 140 matched controls
    Effect: Incidence 11.6% with MgO 900 mg/day or more; 27.0% with eCcr 28.2 mL/min or less; 53.1% with both
    Certainty: One hospital in Japan; observational data.
    Magnes Res, 2023 · checked 2026-10-06 · we read the abstract
14. ev-mgcon-14 · Laboratory or animal data · case series · n = 2
    Herein, we report two cases of hypermagnesemia in patients taking magnesium hydroxide for constipation.
    
    Who: Two patients: 82-year-old woman with end-stage renal disease; 50-year-old woman with normal renal function
    Effect: Serum magnesium 9.9 and 11.0 mg/dL; one cardiac arrest and death
    Certainty: Case series = D until Roman decides; does not show frequency.
    Electrolyte Blood Press, 2023 · checked 2026-10-06 · we read the abstract
15. ev-mgcon-15 · Observational data · non-randomized pharmacokinetic study · n = —
    A healthy volunteer study showed that MgO was also associated with significant decreases of 35.3 and 80.9% in the AUC0-12 of L-dopa and carbidopa, respectively.
    
    Who: Healthy volunteers, open-label non-randomized two-phase study
    Effect: AUC0-12 L-dopa -35.3%; carbidopa -80.9%
    Certainty: Pharmacokinetics, not clinical outcomes; N not named in the abstract.
    Eur J Clin Pharmacol, 2019 · checked 2026-10-06 · we read the abstract
16. mg-09 · Position of an expert body
    High doses of magnesium from dietary supplements or medications often result in diarrhea that can be accompanied by nausea and abdominal cramping.
    
    Who: people taking high-dose supplements
    Effect: diarrhea
    NIH Office of Dietary Supplements, Health Professional Fact Sheet: Magnesium · checked 2026-09-15
17. mg-r6-05 · Randomized controlled trial(s) · prospective, double-blinded, randomized, placebo-controlled trial over 28 consecutive days · n = 90 patients, all of whom completed the study
    The response rate for overall improvement was 11.7% in the placebo group, 69.2% in the senna group, and 68.3% in the MgO group (P < 0.0001). ... The frequency of severe treatment-related adverse events was 0%.
    
    Who: patients with chronic idiopathic constipation, mean age 42 years, 93 percent women, mean symptom duration 9.9 years
    Effect: overall improvement in 68.3 percent on magnesium oxide, 69.2 percent on senna and 11.7 percent on placebo (P < 0.0001); severe treatment-related adverse events 0 percent
    Certainty: 28 days in 90 mostly young women with normal kidneys; the laxative effect is the point of the dose here, so it is not a side effect in this population but is exactly what goes wrong in someone taking magnesium for another reason
    Morishita et al., American Journal of Gastroenterology, 2021 · checked 2026-09-17 · we read the abstract
18. mg-e1-15 · Position of an expert body · perspective article summarizing the properties of the salt forms on sale · n = narrative summary, no pooled estimate
    Mg gluconate and Mg chloride have been preferred for oral replacement because they cause diarrhea less often than other salt forms [47].
    
    Who: adults taking oral magnesium supplements
    Effect: magnesium gluconate and magnesium chloride cause diarrhea less often than other salt forms; magnesium carbonate is avoided as too insoluble to absorb well; citrate is highly water-soluble and may affect stools more than oxide, which is poorly soluble even in acid
    Certainty: a narrative summary with single citations behind each claim, not a head-to-head comparison; our own corpus has no trial comparing oxide, citrate and glycinate directly, which is why this narrative is the best answer available and is graded E
    Costello & Rosanoff, Advances in Nutrition, 2023 · checked 2026-09-18 · we read the fulltext