Magnesium Hydroxide — Veterinary Vademecum Drug Reference

Dosage & Administration Guidelines

Species	Route	Dose	Frequency	Duration & Clinical Notes
Dog	PO	Antacid: 5-10 mL of 8% suspension (or 200-400 mg/kg) q4-6h; Laxative: 10-30 mL of 8% suspension (or 1-2 g per animal) q24h	q4-6h for antacid; q24h for laxative	Duration: As needed for symptom relief; not for long-term use Notes: Use with caution in patients with renal impairment. For constipation, ensure adequate hydration.
Cat	PO	Antacid: 2-5 mL of 8% suspension q4-6h; Laxative: 5-10 mL of 8% suspension q24h	q4-6h for antacid; q24h for laxative	Duration: As needed Notes: Cats are sensitive to magnesium; avoid in renal disease.
Horse	PO	Antacid: 100-200 g per animal q12h; Laxative: 500-1000 g per animal q24h	q12h for antacid; q24h for laxative	Duration: As needed Notes: Not a primary treatment for gastric ulcers; use only as adjunct. Monitor for electrolyte imbalances.
Cattle	PO	Antacid: 200-400 g per animal q12h; Laxative: 500-1000 g per animal q24h	q12h for antacid; q24h for laxative	Duration: As needed Notes: May be used in rumen acidosis to neutralize excess acid. Ensure adequate hydration.
Small Ruminants (sheep, goats)	PO	Antacid: 10-20 g per animal q12h; Laxative: 25-50 g per animal q24h	q12h for antacid; q24h for laxative	Duration: As needed Notes: Use with caution in animals with urinary calculi risk.
Rabbit	PO	1-2 mL of 8% suspension per kg q12h for GI stasis	q12h	Duration: Until GI motility improves Notes: Use only under veterinary supervision; ensure hydration and supportive care.
Birds (e.g., psittacines)	PO	0.1-0.2 mL of 8% suspension per kg q12h	q12h	Duration: As needed Notes: Limited data; use with caution.
Reptiles (e.g., tortoises)	PO	10-20 mg/kg q24h for constipation	q24h	Duration: Until defecation Notes: Ensure adequate hydration and temperature.

Clinical Indications & Species Uses

General Indications

Antacid
Osmotic laxative

Dog (Canine)

Antacid for gastric hyperacidity
Laxative for constipation
Treatment of dietary magnesium deficiency (rare)

Cat (Feline)

Antacid for gastric hyperacidity
Laxative for constipation (less common)

Horse (Equine)

Antacid for gastric ulcers (adjunctive)
Laxative for impaction (rarely used)

Cattle (Bovine)

Antacid for rumen acidosis (adjunctive)
Laxative for indigestion

Small Ruminants (Sheep / Goat)

Antacid for rumen acidosis
Laxative for constipation

Rabbit & Small Mammals

Laxative for gastrointestinal stasis (uncommon)

Avian & Poultry

May be used as a laxative in some species

Exotic & Other Species

Laxative for reptiles (e.g., impaction)

Pharmacology & Mechanism of Action

Drug Class: Antacid, Osmotic Laxative | Pharmacological Group: Magnesium-containing compounds

Mechanism of Action: Magnesium hydroxide acts as an antacid by neutralizing gastric acid, forming magnesium chloride and water. As a laxative, it exerts an osmotic effect in the intestinal lumen, drawing water into the bowel, which increases intraluminal pressure and stimulates peristalsis, leading to evacuation. It may also stimulate the release of cholecystokinin, which enhances intestinal motility and fluid secretion.

Pharmacodynamics: The antacid effect is rapid and of short duration, typically lasting 30-60 minutes. The laxative effect occurs within 2-6 hours after oral administration, depending on the dose and species. Magnesium ions are poorly absorbed from the gastrointestinal tract, but the absorbed fraction can cause systemic effects, particularly in patients with renal impairment.

⚡ Pharmacokinetics Summary

Absorption: Magnesium hydroxide is poorly absorbed from the gastrointestinal tract. Only about 15-30% of the magnesium is absorbed, primarily in the small intestine. The rate of absorption is influenced by gastric emptying and intestinal transit time.

Distribution: Absorbed magnesium is distributed throughout the extracellular fluid and is taken up by bone and intracellular compartments. It crosses the placenta and is excreted in milk.

Metabolism: Magnesium is not metabolized; it exists as the divalent cation Mg2+.

Excretion: The unabsorbed fraction is excreted in the feces. Absorbed magnesium is primarily excreted by the kidneys, with a small amount lost in sweat and milk. In renal impairment, excretion is reduced, leading to accumulation.

Half-Life: The half-life of absorbed magnesium is approximately 4-6 hours in dogs and cats, but may be prolonged in renal insufficiency.

Bioavailability: Oral bioavailability is low, estimated at 15-30%, due to poor absorption and rapid gastrointestinal transit.

Protein Binding: Approximately 30% of magnesium in plasma is protein-bound, mainly to albumin.

Available Formulations & Strengths

Oral Suspension 8% (400 mg/5 mL) (PO)

Oral Tablet 325 mg, 650 mg (PO)

Oral Powder Bulk powder for reconstitution (PO)

Contraindications & Clinical Warnings

Contraindications:

Renal insufficiency or failure
Hyper-magnesemia
Intestinal obstruction or perforation
Appendicitis-like symptoms (in humans; extrapolate to animals)
Dehydration or electrolyte imbalance
Concurrent use with other magnesium-containing products

Warnings & Clinical Precautions:

Use with caution in patients with cardiac disease, as magnesium can affect cardiac conduction.
Prolonged use may lead to hypermagnesemia, especially in renal impairment.
May interfere with absorption of other drugs; separate administration by at least 2 hours.
In food animals, observe withdrawal times.
In neonates and young animals, use with caution due to immature renal function.
Monitor serum magnesium levels in patients on prolonged therapy or with renal disease.

Adverse Effects & Reactions

Common:

Diarrhea
Loose stools
Abdominal cramping
Nausea (in animals, may manifest as inappetence)

Serious / Severe:

Hypermagnesemia (with renal impairment)
Hypotension
Respiratory depression
Cardiac arrhythmias
Muscle weakness

Rare:

Metabolic alkalosis (with prolonged antacid use)
Fecal impaction (if inadequate fluid intake)

Key Drug Interactions

Interacting Drug / Class	Clinical Effect & Mechanism	Severity
Tetracyclines	Magnesium binds to tetracyclines, reducing their absorption and efficacy.	High
Fluoroquinolones	Reduced absorption of fluoroquinolones due to chelation with magnesium.	High
Digoxin	Magnesium may affect GI motility, potentially altering digoxin absorption; also, hypomagnesemia can increase digoxin toxicity, but hypermagnesemia may have additive effects on cardiac conduction.	Moderate
Iron supplements	Reduced absorption of iron due to chelation.	Moderate
Mycophenolate mofetil	Reduced absorption of mycophenolate.	Moderate
Corticosteroids	May increase magnesium loss, potentially counteracting the effects.	Mild

Overdose & Toxicity Management

Signs of Toxicity:

Diarrhea
Lethargy
Muscle weakness
Hypotension
Bradycardia
Respiratory depression
Coma
Cardiac arrest

Emergency Treatment Protocol: Discontinue the drug. Provide supportive care including IV fluids to promote renal excretion. In severe cases, administer calcium gluconate IV to antagonize the effects of magnesium. In cases of renal failure, dialysis may be necessary. Monitor serum magnesium levels and cardiac function.

Food Animal Withdrawal Times

🥩 Meat: 0 days🥛 Milk: 0 days🥚 Eggs: 0 days

Magnesium hydroxide is generally considered to have zero withdrawal times in food animals when used according to label, but extra-label use may require extended withdrawal periods. Consult a veterinarian for specific withdrawal times.

Storage, Handling & Regulatory Information

Storage Temperature: Store at controlled room temperature (15-30°C).

Handling & Special Conditions: Keep container tightly closed. Protect from freezing. Shake suspension well before use.

Dispensing Status: Over-The-Counter (OTC)

Approval Status: Not FDA-approved for veterinary use; used off-label.

Extra-Label (Off-Label) Use: In the US, magnesium hydroxide is an OTC product for human use, but in veterinary medicine it may be used extra-label under AMDUCA. For food animals, extra-label use requires a valid veterinarian-client-patient relationship and extended withdrawal times.

Clinical Pearls & Practice Notes

💡 Clinical Insights: Magnesium hydroxide is a useful adjunct for managing gastric hyperacidity and constipation in various species. However, its use in veterinary medicine is largely empirical and based on human data. It is essential to consider the patient's renal function, as magnesium accumulation can lead to serious toxicity. For constipation, it is crucial to ensure adequate hydration and to rule out obstructions. In food animals, it may be used to manage rumen acidosis, but other treatments (e.g., sodium bicarbonate) are often preferred. Always separate administration from other oral medications by at least 2 hours to avoid drug interactions. Monitor for adverse effects, especially in debilitated or very young animals.

References & Literature

📚 Plumb's Veterinary Drug Handbook
📚 Papich Veterinary Pharmacology and Therapeutics
📚 Compendium of Veterinary Products (CVP)