LACTOLOGY FOUNDATION

Magnesium hydroxide (ATC A02AA04) is a locally acting antacid and osmotic laxative, which is considered compatible with breastfeeding when used at recommended doses and for short periods in healthy, term infants. The toxicological risk during lactation is very low because systemic absorption in the mother is minimal, transfer into milk is small, and oral bioavailability for the breastfed infant is poor.

Substance and ATC classification

Magnesium hydroxide is an inorganic magnesium salt used as an antacid and saline laxative. It is classified in the ATC system as A02AA04, within the group of magnesium compounds used as plain antacids. For adult oral therapy as an antacid, the defined daily dose (DDD) is 3 g by mouth.

Mechanism of action

As an antacid, magnesium hydroxide neutralizes gastric hydrochloric acid, forming magnesium chloride and water and thereby reducing acidity and acid‑mediated mucosal irritation. As a laxative, it acts as an osmotic agent in the intestinal lumen, retaining water with the stool, increasing stool volume, and stimulating peristalsis, typically producing a bowel movement within 30 minutes to 6 hours after ingestion.

Clinical indications

Magnesium hydroxide is indicated for the short‑term relief of indigestion, heartburn, hyperacidity, flatulence, and related upper gastrointestinal discomfort. In higher doses it is used as a saline laxative for occasional constipation and for bowel evacuation before diagnostic procedures or surgery. These indications apply to both prescription and non‑prescription “milk of magnesia” preparations.

Dosage and administration

Oral liquid, suspension and tablet formulations are available over the counter in a range of strengths. Typical adult doses are 5–15 ml up to four times daily when used as an antacid, and 15–60 ml once daily (often at bedtime, with adequate fluid) when used as a laxative, with dose adjustments and specific limits in children according to age. Therapy should be short term, and persistent symptoms require medical evaluation.

Pharmacokinetics and systemic exposure

After oral administration, only a small fraction of magnesium is absorbed from the gastrointestinal tract in individuals with normal intestinal mucosa. Unabsorbed magnesium hydroxide and its salts are eliminated in the feces, while the absorbed fraction is excreted in the urine, provided renal function is normal. The low systemic bioavailability underlies the low systemic toxicity profile in patients without renal impairment.

General adverse effects and toxicity

At therapeutic doses, the most frequent adverse effects are gastrointestinal, including abdominal cramping, nausea and dose‑dependent diarrhea. With chronic or excessive use, fluid and electrolyte disturbances and laxative dependence may develop. In overdose or in patients with significant renal impairment, magnesium accumulation can lead to hypermagnesemia, with symptoms such as flushing, hypotension, muscle weakness, respiratory depression, arrhythmias, and in extreme cases coma and cardiac arrest.

Contraindications and cautions

Magnesium hydroxide is contraindicated in patients with renal failure, intestinal obstruction or perforation, acute surgical abdomen, severe abdominal pain of unknown origin, and in those with known hypersensitivity to magnesium salts. Caution is required in patients with dehydration, colitis, rectal bleeding, heart block, or other conditions where fluid shifts and electrolyte changes may be poorly tolerated.

Drug–drug and drug–nutrient interactions

Like other antacids, magnesium hydroxide may alter gastric pH and gastrointestinal transit time, potentially affecting the absorption of concomitantly administered medicines. Interactions have been reported with drugs such as dicoumarol, cimetidine and ibuprofen, and antacids in general can reduce or delay absorption of certain antibiotics and other pH‑sensitive medicines, so separation of administration times is recommended.

Use in pregnancy

Magnesium crosses the placenta and appears in fetal circulation, but clinical formulations of magnesium hydroxide are generally considered low risk when used intermittently and at recommended doses. Product information often advises that use in pregnancy (and lactation) should be on medical advice and limited to the lowest effective dose for the shortest possible duration.

Pharmacology in lactation

Magnesium is a normal constituent of human milk, with mean baseline concentrations around 31 mg/L and a reported range of about 15–64 mg/L. Oral ingestion of magnesium salts, including magnesium hydroxide, does not significantly increase milk magnesium levels because of poor maternal gastrointestinal absorption and homeostatic renal excretion. Intravenous magnesium therapy causes only small, transient increases in milk magnesium concentration, which still result in minimal infant systemic exposure.

Transfer into breast milk and infant exposure

Data from LactMed and other lactation pharmacology sources indicate that a study of magnesium hydroxide use during breastfeeding showed no adverse reactions in breastfed infants. Only small amounts of magnesium appear in breast milk after maternal exposure, and the additional contribution from magnesium hydroxide is negligible compared with physiological milk content. Furthermore, oral absorption of magnesium by the healthy term infant is low, so any ingested excess is largely excreted in the stool.

Toxicological risk for the breastfed infant

For healthy term infants, the toxicological risk from maternal use of magnesium hydroxide at recommended antacid or occasional laxative doses is considered very low. Major references classify magnesium hydroxide as “compatible” or “very low risk” in breastfeeding, and no specific precautions or breastfeeding interruptions are required under usual use. The primary theoretical concern is excessive maternal dosing or chronic use leading to maternal hypermagnesemia, which in turn could slightly increase infant exposure, although clinically significant toxicity in this scenario has not been documented.

Special populations and higher‑risk scenarios

The risk profile may differ in certain vulnerable groups. Premature infants and neonates may exhibit higher intestinal permeability and magnesium absorption, so in these infants a cautious approach and closer clinical observation are advisable if the mother uses magnesium hydroxide repeatedly. Similarly, if the breastfed infant has renal impairment, even small increases in absorbed magnesium could pose a higher toxicological risk, and alternative maternal therapies might be preferable.

Maternal safety considerations during lactation

From the maternal perspective, the main safety considerations during lactation are the same as outside lactation: avoidance of prolonged, high‑dose use and careful use in women with renal dysfunction or on interacting medications. Excessive laxative doses can cause significant diarrhea and electrolyte imbalance in the mother, which may indirectly affect lactation through dehydration and reduced well‑being. Women should be advised not to exceed labelled doses or recommended treatment duration without medical review.

Practical recommendations for breastfeeding women

For breastfeeding mothers with occasional dyspepsia or constipation, short‑term, on‑label use of magnesium hydroxide is acceptable without the need to interrupt breastfeeding. It is prudent to avoid chronic daily use, high doses beyond recommendations, and combination with other magnesium‑containing laxatives or antacids to minimize the risk of maternal and infant magnesium overload. In high‑risk infants (preterm, renal disease) or when the mother has significant renal impairment, clinicians should individualize therapy and consider alternative treatments with more extensive lactation safety data if necessary.