Magnesium Hydroxide

Magnesium hydroxide is a magnesium salt used orally as an antacid and saline cathartic. Its clinical effect depends on the amount delivered and the site of action. Veterinary applications include selected constipation cases and antacid treatment in the stomach or rumen, particularly in large-animal practice.

Group

Osmotic laxative and antacid

Mechanism of Action

Magnesium hydroxide reacts with gastric acid and consumes hydrogen ions, increasing luminal pH. Magnesium salts remaining in the intestine retain water through an osmotic effect, softening contents and promoting bowel evacuation. In the rumen, acid neutralization can raise ruminal pH. These local actions account for its antacid and cathartic applications.

Dose Rates

Dogs

Use / stage	Dose	Route	Schedule	Instructions
Antacid	400–800 mg/animal	PO	Every 4–6 h	Equivalent to 5-10 mL of the 400 mg/5 mL suspension.
Cathartic	1200–4000 mg/animal	PO	Every 24 h	Equivalent to 15-50 mL of 400 mg/5 mL suspension.
GI-disease-associated magnesium replacement	400–1200 mg/animal	PO	Every 24 h	Equivalent to 5-15 mL/day of 80 mg/mL milk of magnesia; reassess magnesium, calcium and stool tolerance.
Antacid	400–2400 mg/animal	PO	Every 12–24 h	Separate 5-30 mL alternative for 80 mg/mL product.
Cathartic	80–1600 mg/animal	PO		Equivalent to the listed 1-20 mL milk-of-magnesia range; no repeat frequency is supplied.

Cats

Use / stage	Dose	Route	Schedule	Instructions
Antacid	400–800 mg/animal	PO	Every 4–6 h	Equivalent to 5-10 mL of the 400 mg/5 mL suspension.
Cathartic	160–480 mg/animal	PO	Every 24 h	Equivalent to 2-6 mL of 400 mg/5 mL suspension; oral route is established by the monograph context.
Antacid	400–1200 mg/animal	PO	Every 12–24 h	Separate 5-15 mL alternative for 80 mg/mL product.
Cathartic	80–400 mg/animal	PO		Equivalent to 1-5 mL milk of magnesia; no repeat frequency is supplied.

Sheep

Use / stage	Dose	Route	Schedule	Instructions
Antacid and cathartic	1 g/kg	PO	Once	Use the mass instruction rather than the conflicting adult bolus-count example.

Cattle

Use / stage	Dose	Route	Schedule	Instructions
Antacid and cathartic	1 g/kg	PO	Once	Use the mass instruction rather than the conflicting adult bolus-count example.
Documented rumen acidosis	Reference only	PO		Adult: up to 1 g/kg mixed in 2-3 gallons warm water by stomach tube; smaller repeats may be considered in 6-12 hours. If rumen contents were evacuated, initial total must not exceed 225 g. Correct dehydration and systemic acidosis concurrently. Calf and sheep/goat directions use 1/8-1/4 of the adult amount but lack a complete independent weight-based prescription.

Indications

Short-term cathartic treatment of selected constipation in veterinary patients.
Bowel evacuation before selected veterinary procedures.
Antacid treatment of selected gastric acid-related conditions.
Ruminal antacid treatment in selected cattle with excessive ruminal acidity, within an appropriate management plan.

Contraindications

Renal impairment may cause magnesium accumulation.
Dehydration
In ruminants restrict clinical antacid use to documented rumen acidosis rather than undiagnosed rumen dysfunction; avoid magnesium-containing antacids with renal disease.

Interactions

Decreases absorption of fluoroquinolones and other drugs susceptible to magnesium chelation.

Adverse effects

Diarrhoea, dehydration and electrolyte losses
Hypermagnesaemia

Toxicity

Stop treatment if weakness or excessive diarrhoea occurs; assess magnesium and renal function.

Preparations

Preparation	Form	Strength	Route
Milk of Magnesia 400 mg/5 mL	Oral Liquid	400 mg / 5 mL	PO
Magnesium hydroxide 27 g bolus	Tablet	27 g / tablet	PO
Magnesium hydroxide 90 g bolus	Tablet	90 g / tablet	PO
Magnesium hydroxide concentrated 160 mg/mL	Oral Liquid	160 mg / mL	PO

Monitoring and Administration

Administer only to adequately hydrated patients and monitor electrolytes with repeated use.

The powder concentration and its stated equivalence are inconsistent; do not substitute powder volumes without the actual product composition.

The powder-mixing example uses 0.45 kg powder in 1 gallon water, administering 500 mL per 45 kg. This is a product recipe, not a verified pure-ingredient concentration. Polyox volume instructions are 180 mL below 225 kg or 360 mL above 225 kg, but composition is not supplied.

Foal ulcer adjunct uses 15 mL of a specific aluminium/magnesium hydroxide suspension four times daily; ingredient concentrations are not given, so this cannot be treated as magnesium hydroxide alone. Store milk of magnesia below 35°C and do not freeze.