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Osmotic laxative / antacid Pregnancy: §4.6 (verbatim, the whole section): 'As with all other medicines, use of this product should be avoided in the above conditions unless recommended by a doctor.' The retrieved text does not say which 'above conditions' are meant and gives no pregnancy or lactation statement beyond this.

Magnesium hydroxide

Brand names: Milk of Magnesia

Magnesium hydroxide is a magnesium salt used as an antacid for dyspepsia and as an osmotic laxative for constipation.

Adult dose

Dose: Adults, the elderly and children over 12 years — as an antacid (symptoms of flatulence, heartburn, dyspepsia and indigestion): one to two 5 ml spoonfuls in a little water when required. As a laxative (relief of occasional constipation): 25 - 50 ml (five to ten 5 ml spoonfuls) in a glass of water at bedtime
Route: Oral (Magnesium Hydroxide Mixture BP) — shake the bottle
Frequency: Antacid: when required; laxative: at bedtime
Pinned product: Magnesium Hydroxide Mixture BP (oral). §4.1 verbatim: '1. For relief of the symptoms of flatulence, heartburn, dyspepsia and indigestion 2. As a laxative for relief of occasional constipation' — covers BOTH of the page's uses; route oral matches. §4.2 verbatim: '1. As an antacid Adults, the elderly and children over 12 years: one to two 5ml spoonfuls in a little water when required. Children under 12 years: not suitable. 2. As a laxative Adults, the elderly and children over 12 years: 25 - 50ml (five to ten 5ml spoonfuls) in a glass of water at bedtime. Children under 12 years: not suitable. Directions for use: Shake the bottle.' The label prints NO maximum daily dose, so none is given. STRENGTH: the retrieved sections do not state the milligram strength of magnesium hydroxide per 5 ml, so the volumes apply to this UK mixture only and must not be applied to other milk of magnesia / magnesium hydroxide preparations. Children under 12: not suitable (see paediatric note) — Children: use a children's formulary. Cautions (§4.4): the elderly and debilitated; impaired liver function; impaired kidney function (hypermagnesaemia may result); 'If constipation persists consult your doctor'; discard any unused mixture 3 months after opening. Antacid spacing (§4.5): drug administrations should be separated by approximately 2 to 3 hours.

Dose adjustments

Renal

No numeric renal dose adjustment is printed. §4.3: contraindicated in severe renal failure. §4.4: use with caution in patients with impaired kidney function as hypermagnesaemia may result. §4.8: hypermagnesaemia observed after prolonged administration to patients with renal impairment.

Hepatic

No numeric hepatic dose adjustment is printed. §4.4: use with caution in patients with impaired liver function.

Dose independently re-sourced from UK Summary of Product Characteristics (SPC) via the eMC; US FDA prescribing information (openFDA / DailyMed) — cross-check; US labelling may differ from UK — clinician-reviewed. Always confirm against the product SmPC and your local formulary before prescribing.

Contraindications

  • Severe renal failure
  • Acute gastrointestinal conditions
  • Hypersensitivity to magnesium salts or any of the other ingredients
  • Not suitable for children under 12 years (§4.2, §4.4)

Side effects

  • Hypermagnesaemia — observed after prolonged administration of magnesium hydroxide to patients with renal impairment
  • Abdominal pain
  • Diarrhoea (a dose-dependent effect)
  • Colic
  • In young children magnesium hydroxide can produce hypermagnesaemia, especially if they have renal impairment or dehydration (§4.4)

Interactions

  • As a precaution for antacids, separate drug administrations by approximately 2 to 3 hours to minimise interactions affecting pharmacokinetics (§4.5)
  • Magnesium salts reduce the absorption of a number of other drugs, including ACE inhibitors (captopril, enalapril, fosinopril), antibacterials and antifungals (e.g. azithromycin, cefaclor, cefpodoxime, isoniazid, itraconazole, nitrofurantoin, rifampicin, tetracyclines, ketoconazole, quinolones), antivirals (atazanavir, fosamprenavir, tipranavir), fexofenadine, bisphosphonates, deflazacort, digoxin, dipyridamole, diflunisal, gabapentin, phenytoin, lansoprazole, levothyroxine, mycophenolate, iron preparations, rosuvastatin, sulpiride, phenothiazines, chloroquine, hydroxychloroquine, proguanil and penicillamine (§4.5)
  • Sodium polystyrene sulphonate — concomitant use may produce metabolic alkalosis (§4.5)
  • Aspirin / salicylates — alkaline urine due to some antacids increases excretion (§4.5)
  • Nilotinib — antacids should be avoided (§4.5)

Monitoring

  • Patients with impaired kidney function — hypermagnesaemia may result (§4.4)
  • Young children, especially with renal impairment or dehydration — risk of hypermagnesaemia (§4.4)
  • If constipation persists consult a doctor (§4.4)

Clinical monograph

How it works

It neutralises gastric acid to relieve acid-related symptoms and, in the bowel, acts osmotically to retain water and stimulate evacuation.

Prescribing in practice

  • Magnesium can accumulate and cause hypermagnesaemia in renal impairment, so use with caution and avoid in significant renal failure.
  • Its laxative effect means it is frequently combined with aluminium-containing antacids to balance bowel effects.
  • Antacid use can reduce the absorption of other medicines, so separate administration from interacting drugs.

Monitoring

Routine monitoring is not required for occasional use, but assess renal function in patients with impairment or with regular magnesium intake.

Counselling the patient

  • It can loosen the bowels; leave a gap between this and your other medicines.
  • Tell your clinician if you have kidney problems or if symptoms persist.

Evidence & guidelines

Magnesium hydroxide is an established antacid and osmotic laxative, with use in constipation and dyspepsia supported by NICE guidance.

Reference: Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. The structured dose values shown have been reviewed by a clinician. Monograph status: clinician-reviewed (2026-07-04).

Related

Curated clinical cross-links plus same-class fallbacks.

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