Magnesium hydroxide (milk of magnesia)
Milk of magnesia works as an osmotic laxative for occasional constipation: a US label says it usually produces a bowel movement in half an hour to six hours, and a systematic review of over-the-counter laxatives graded the evidence for magnesium salts as moderate, behind polyethylene glycol and senna. It is also sold as an antacid, but we found no trial of magnesium hydroxide alone for heartburn.
The one serious caution is kidney impairment, because magnesium can build up to dangerous levels (hypermagnesaemia), and it should be kept apart from other medicines, including some antibiotics.
At a glance
Last reviewed 10 October 2026What it's used for
Evidence is rated per use. The same ingredient can be strong for one thing and traditional for another.
| Use | Evidence | What the research shows |
|---|---|---|
| Occasional constipation in adults (laxative)Compare remedies for digestion → | Moderate | A 2021 systematic review of over-the-counter therapies for chronic constipation (41 randomised trials of at least 4 weeks) found good evidence for polyethylene glycol and senna and moderate evidence for magnesium salts, with diarrhoea, nausea, bloating and abdominal pain as the common adverse effects. That evidence is for magnesium salts as a group, not magnesium hydroxide specifically, and the review cites small samples and varied designs as limits. The US label for milk of magnesia lists relief of occasional constipation, with a bowel movement generally in half an hour to six hours.1,4 |
| Constipation in children (laxative) | Limited | A Cochrane review of 25 trials (2,310 children) found polyethylene glycol gave slightly more stools a week than milk of magnesia (3 studies, 211 children, mean difference 0.69, 95% CI 0.48 to 0.89), a gap the authors said may not be clinically significant, and overall evidence quality was low, with 14 trials at high risk of bias. In one 12-month open-label trial of 83 children aged 6 months to 18 years, success was similar with magnesium hydroxide (40 of 42) and polyethylene glycol (40 of 41), but children over 4 rejected magnesium hydroxide more often.2,3 |
| Heartburn and indigestion (antacid)Compare remedies for acid reflux & heartburn → | Traditional | Magnesium hydroxide is a long-standing OTC antacid: the US label lists relief of heartburn, sour stomach, acid indigestion and upset stomach, and the NHS names magnesium hydroxide among antacid ingredients that relieve symptoms for a few hours without treating the cause. We did not find a trial of magnesium hydroxide alone for heartburn, so we rate this 1, on regulatory use rather than clinical research. Its laxative effect can be an unwanted side effect here.4,5 |
| Too much magnesium (harm signal), especially with kidney impairment | Limited | Not a use but the main safety issue. A 2018 review says magnesium-containing laxatives can cause hypermagnesaemia, particularly when kidney function is reduced. In a Japanese hospital study of 193 people taking daily magnesium oxide (a related laxative, not hydroxide), 10 (5.2%) had hypermagnesaemia, linked to stage 4 chronic kidney disease and to dose. A case report describes a fatal case in a woman with normal kidney function who had used magnesium laxatives regularly for constipation. Evidence is case-level and observational, so the size of the risk is unknown.6,7,8 |
How much
Ranges below are what studies used and what supplements commonly contain. They are not a recommendation for you. Start at the low end.
How people take it
Safety
Who should avoid
- Kidney disease — magnesium can build up (hypermagnesaemia); the label says ask first
- Doctor-advised low-magnesium diet — the label says ask first
- Abdominal pain, nausea or vomiting — the label says ask a doctor before using as laxative
- Sudden bowel change lasting over 2 weeks, or rectal bleeding — see a doctor
- Pregnancy or breastfeeding — the label says ask a health professional
- Children under 12 — the label says ask a doctor
- Needing a laxative for more than a week — the label says stop and ask a doctor
Known interactions
Questions people ask
What are the side effects of magnesium hydroxide?
The NHS lists diarrhoea or constipation, wind, stomach cramps and feeling sick for antacids, and a laxative effect is expected with milk of magnesia.
The serious risk is too much magnesium in the blood, mostly in people with reduced kidney function (2018 review; 5.2% of 193 people on daily magnesium oxide), so ask a doctor before using it if you have kidney disease.
What is milk of magnesia used for?
Two things, both on the US label: occasional constipation, where it draws water into the bowel as a saline laxative, and heartburn or indigestion as an antacid. The laxative use has moderate evidence for magnesium salts in a 2021 review; we found no trial of magnesium hydroxide alone as an antacid.
Is this the same as magnesium supplements for sleep or anxiety?
No. Magnesium hydroxide is used as a laxative and antacid, where it acts in the gut. Magnesium taken as a supplement for sleep, stress or anxiety is covered on our magnesium glycinate page, and that evidence is about other magnesium forms and a different purpose, so we do not repeat it here.
Can I take it with my other medicines?
Not at the same time. The NHS says antacids can affect how well other medicines work and not to take other medicines within 2 to 4 hours of one. Magnesium and aluminium hydroxide antacids cut absorption of some quinolone antibiotics by 50 to 90%, so ask a pharmacist about timing for your specific medicine.
Sources (9)
- Systematic review Rao SSC, Brenner DM. Efficacy and Safety of Over-the-Counter Therapies for Chronic Constipation: An Updated Systematic Review. Am J Gastroenterol. 2021.
- Systematic review Gordon M, MacDonald JK, Parker CE, Akobeng AK, Thomas AG. Osmotic and stimulant laxatives for the management of childhood constipation. Cochrane Database Syst Rev. 2016.
- Randomised controlled trial Worona-Dibner L, Vázquez-Frias R, Valdez-Chávez L, Verdiguel-Oyola M. Efficacy, safety, and acceptability of polyethylene glycol 3350 without electrolytes vs magnesium hydroxide in functional constipation in children from six months to eighteen years of age: A controlled clinical trial. Rev Gastroenterol Mex. 2023.
- Regulator label DailyMed. Milk of Magnesia (magnesium hydroxide) suspension, PAI Holdings LLC dba PAI Pharma: OTC drug label. Updated January 2026.
- Health service guidance NHS. Antacids. Medicines.
- Review William JH, Richards K, Danziger J. Magnesium and Drugs Commonly Used in Chronic Kidney Disease. Adv Chronic Kidney Dis. 2018.
- Cross-sectional study Mori H, Suzuki H, Hirai Y, et al. Clinical features of hypermagnesemia in patients with functional constipation taking daily magnesium oxide. J Clin Biochem Nutr. 2019.
- Case report Bokhari SR, Siriki R, Teran FJ, Batuman V. Fatal Hypermagnesemia Due to Laxative Use. Am J Med Sci. 2018.
- Review Gugler R, Allgayer H. Effects of antacids on the clinical pharmacokinetics of drugs. An update. Clin Pharmacokinet. 1990.
Last reviewed 10 October 2026 · Next review April 2027 · How we rate evidence
Reader notes
Experiences and questions from readers, read by an editor before they appear. They are personal accounts, not evidence, and are not medical advice.