Magnesium hydroxide

Magnesium-based antacid and osmotic laxative

ATC code: A02AA04

Brand names: Milk of Magnesia, Phillips

Updated 24 Sep 2026 · Methodology

Indications

Antacid therapy for mild dyspepsia

First line

Magnesium hydroxide neutralizes stomach acid within minutes, so it is used for occasional heartburn and acid regurgitation. The AEMPS ficha técnica sets the dose precisely: adults and adolescents from 12 years take 2.5 to 5 ml per intake, up to 12.5 ml a day. An antacid is not taken for longer than two weeks without a doctor – persistent heartburn means reflux disease, sometimes an ulcer or a tumour, and guideline CG184 asks for the cause to be worked out in such cases rather than the symptom muffled. Magnesium antacids have their own quirk: they loosen the bowel, and with frequent use diarrhea becomes the main complaint. The product is contraindicated in severe renal impairment – magnesium accumulates and causes hypermagnesemia.

Sources

ICD-10: K30 · ICD-11: DD90.3

Occasional constipation

Second line

At laxative doses magnesium hydroxide works osmotically: it pulls water into the bowel, the contents soften and a stool usually arrives within 6 to 8 hours. The AEMPS ficha técnica allows adults and adolescents from 12 years 5 to 10 ml and up to 25 ml a day, in one or two intakes. The condition is the same as for any laxative: a short course. The ficha asks for assessment if constipation has not resolved within 7 days and recalls that chronic constipation without an organic cause is managed differently – fibre, water, movement, routine. The contraindications are worth going through in advance: severe renal impairment, signs of appendicitis, bowel obstruction, undiagnosed bleeding, ulcerative colitis, diverticulitis, colostomy and ileostomy, chronic diarrhea.

Sources

AEMPS: Ficha técnica de hidroxido de magnesio (2023)

ICD-10: K59.0

Marketing claims without evidence base

The drug is promoted for these uses outside international guidelines. Each entry below is analyzed against AEMPS, FDA, EMA, Cochrane and major RCTs.

Body detoxification and cleansing

Not recommended

Magnesia has long been sold as a way to «cleanse the body»: a liver flush, a bowel clean-out before a diet, a course «to remove toxins». Magnesium hydroxide does none of that. It neutralizes stomach acid and pulls water into the bowel, and that is all. The liver and kidneys filter the blood by themselves, and no pharmacy substance speeds that up. The price of the course, though, is real: taking an osmotic laxative for days loses fluid, sodium and potassium, and in people with impaired kidney function it causes hypermagnesemia. The AEMPS ficha técnica asks plainly to avoid chronic use. If magnesia was recommended as a course to cleanse the body or the liver, consider seeking a second opinion.

Sources

AEMPS: Ficha técnica de hidroxido de magnesio (2023)

How to read tiers A–F

AStrong evidence – first or second line in international guidelines
BRecommended in selected cases
CStudy data contradict each other
DBenefit not proven, no documented harm
FDocumented harm, or risk without benefit

Mechanism of action

Magnesium hydroxide has two actions and both depend on the dose. In the stomach it neutralizes hydrochloric acid: the AEMPS ficha técnica gives the figure – 2.5 ml of suspension binds 17.95 mEq of acid. In the bowel it is absorbed poorly and slowly, so it draws water into the lumen, the volume of contents rises, the wall stretches, peristalsis increases and an osmotic laxative results. Hence the different doses: 2.5 to 5 ml per intake as an antacid, 5 to 25 ml a day as a laxative. Some magnesium does reach the blood, and in people with renal failure it accumulates: hypermagnesemia is the most serious reaction listed in the ficha.

Safety

Contraindications

Hypersensitivity to magnesium hydroxide
Severe renal impairment and fluid or electrolyte imbalance
Signs of appendicitis
Bowel obstruction
Gastrointestinal or rectal bleeding of unknown cause
Ulcerative colitis, diverticulitis, colostomy, ileostomy
Chronic diarrhea

Serious adverse effects

Hypermagnesemia with prolonged use, above all in people with renal failure
Dehydration and electrolyte loss when the laxative dose is misused

Common adverse effects

Diarrhea
Abdominal pain

Pregnancy: can you take Magnesium hydroxide?

Used with caution in pregnancy: high doses and long courses are avoided because of hypermagnesemia risk. A single antacid dose for heartburn in pregnancy is considered acceptable.

Breastfeeding: can you take Magnesium hydroxide?

Magnesium passes into breast milk in small amounts and no changes in infants have been observed. There is no contraindication while breastfeeding, though excessive doses and long courses are avoided.

Calculate RID

Reference information, not a clinical decision. Discuss feeding pauses or changes with your physician or an IBCLC.

Label mentions18 mentions

Verbatim lines from the interaction sections, in the language of the label

From other drugs’ labels

MajorPhenytoin
«Drugs that Affect Phenytoin Concentrations: Antacids Antacids may affect absorption of phenytoin. Calcium carbonate, aluminum hydroxide, magnesium hydroxide Prevention or Management: Phenytoin and…»
FDA, section 7 · Open on DailyMed
MajorRiociguat
«Pharmacokinetic Interactions with Adempas: Antacids: Antacids such as aluminum hydroxide/magnesium hydroxide decrease riociguat absorption and should not be taken within 1 hour of taking Adempas [see…»
FDA, section 7 · Open on DailyMed
MajorSotalol
«Antacids: Avoid administration of oral sotalol within 2 hours of antacids containing aluminum oxide and magnesium hydroxide.»
FDA, section 7 · Open on DailyMed
ModerateLevothyroxine sodium
«Drugs Known to Affect Thyroid Hormone Pharmacokinetics: Proton Pump Inhibitors Sucralfate Antacids (e.g., aluminum & magnesium hydroxides, simethicone) Gastric acidity is an essential requirement for…»
FDA, section 7 · Open on DailyMed
ModerateRosuvastatin
«Rosuvastatin Effects on Other Drugs: See full prescribing information for details regarding concomitant use of rosuvastatin with other drugs that increase the risk of myopathy and rhabdomyolysis. (…»
FDA, section 7 · Open on DailyMed
MinorOlmesartan medoxomil
«Después del tratamiento con un antiácido (hidróxido de magnesio y aluminio) se observó una leve reducción de la biodisponibilidad de olmesartán. La administración conjunta de warfarina y de digoxina…»
AEMPS, section 4.5 · Open on CIMA
Atomoxetine
«Medicamentos que actúan sobre el pH gástrico: Los medicamentos que elevan el pH gástrico (hidróxido de magnesio/hidróxido de aluminio, omeprazol) no tuvieron efecto alguno sobre la biodisponibilidad…»
AEMPS, section 4.5 · Open on CIMA
Atomoxetine
«Drugs that Affect Gastric pH: Drugs that elevate gastric pH (magnesium hydroxide/aluminum hydroxide, omeprazole) had no effect on atomoxetine bioavailability.»
FDA, section 7 · Open on DailyMed
Azithromycin
«Recomendación relativa a la administración conjunta: No se observaron cambios clínicamente significativos en la exposición de azitromicina o de los medicamentos administrados conjuntamente en los…»
AEMPS, section 4.5 · Open on CIMA
Cefditoren pivoxil
«Antiácidos: La administración simultánea de antiácidos conteniendo hidróxido de magnesio y aluminio y»
AEMPS, section 4.5 · Open on CIMA
Cobicistat
«Established and Other Potentially Significant Interactions: Antacids: e.g., aluminum and magnesium hydroxide (please also see H2-Receptor Antagonists and Proton Pump Inhibitors below) ↓ atazanavir…»
FDA, section 7 · Open on DailyMed
Dofetilide
«Drug-Drug Interactions: Cimetidine: (see WARNINGS, CONTRAINDICATIONS ) Concomitant use of cimetidine is contraindicated. Cimetidine at 400 mg BID (the usual prescription dose) co-administered with…»
FDA, section 7 · Open on DailyMed
Efavirenz
«Drugs Without Clinically Significant Interactions with Efavirenz: No dosage adjustment is recommended when efavirenz is given with the following: aluminum/magnesium hydroxide antacids, azithromycin…»
FDA, section 7 · Open on DailyMed
Prednisone
«- Antiácidos (ej. hidróxido de magnesio e hidróxido de aluminio): la administración concomitante puede reducir la absorción de glucocorticoides, lo que comportaría una reducción de la eficacia de…»
AEMPS, section 4.5 · Open on CIMA
Roflumilast
«La coadministración con un antiácido (combinaciones de hidróxido de aluminio e hidróxido de magnesio) no alteró la absorción ni la farmacocinética de roflumilast ni de su N-óxido.»
AEMPS, section 4.5 · Open on CIMA
Tacrolimus
«Recomendaciones relativas a la a dministración simultánea: Agentes procinéticos: metoclopramida, cimetidina e hidróxido de magnesio-aluminio»
AEMPS, section 4.5 · Open on CIMA
Tadalafil
«Potential for Other Drugs to Affect Tadalafil Tablets: Antacids — Simultaneous administration of an antacid (magnesium hydroxide/aluminum hydroxide) and tadalafil reduced the apparent rate of…»
FDA, section 7 · Open on DailyMed
Vardenafil
«Estudios in vivo: La farmacocinética de vardenafilo (20 mg) no se vio afectada por la administración conjunta con el antagonista H 2 , ranitidina (150 mg dos veces al día), digoxina, warfarina…»
AEMPS, section 4.5 · Open on CIMA

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Frequently asked

What is Magnesium hydroxide used for?

Magnesium hydroxide is evaluated for the following indications with varying evidence strength: Antacid therapy for mild dyspepsia (evidence tier B), Occasional constipation (evidence tier B), Body detoxification and cleansing (evidence tier F). See the full indication matrix with dosing and citations above on this page.

What are the side effects of Magnesium hydroxide?

Common side effects of Magnesium hydroxide (≥ 1 in 100): Diarrhea, Abdominal pain. See the Safety section for uncommon and serious reactions.

Is Magnesium hydroxide safe during pregnancy?

Used with caution in pregnancy: high doses and long courses are avoided because of hypermagnesemia risk. A single antacid dose for heartburn in pregnancy is considered acceptable.

Is Magnesium hydroxide compatible with breastfeeding?

Magnesium passes into breast milk in small amounts and no changes in infants have been observed. There is no contraindication while breastfeeding, though excessive doses and long courses are avoided.

Who should not take Magnesium hydroxide?

Magnesium hydroxide is contraindicated in: Hypersensitivity to magnesium hydroxide; Severe renal impairment and fluid or electrolyte imbalance; Signs of appendicitis; Bowel obstruction; Gastrointestinal or rectal bleeding of unknown cause. Full list in the Safety section.

Other drugs in this group

Mastic gum, Omeprazole, Pantoprazole, Rabeprazole, Almagate, Esomeprazole, Slippery elm (inner bark), Sodium alginate