Magnesium Hydroxide
**Magnesium hydroxide** is an oral antacid and laxative used in veterinary medicine. * **Small Animals**: Historically used for esophagitis, gastric hyperacidity, and peptic ulcers, though largely replaced by more effective H2-blockers (e.g., famotidine) and proton-pump inhibitors (e.g., omeprazole). It is still occasionally used as an osmotic laxative (Milk of Magnesia). * **Ruminants**: Highly effective as a rumen alkalinizing agent and laxative for the treatment of **rumen overload syndrome** (grain overload, rumen acidosis). Powder formulations are significantly more effective than boluses for this indication. > **Clinical Pearl**: Magnesium-containing antacids often cause diarrhea, whereas aluminum-containing antacids cause constipation. Many commercial products combine both to balance these effects.
Mechanism: Acts locally in the gastrointestinal tract: * **Antacid**: Neutralizes gastric hydrochloric acid (HCl) โ raises gastric pH. Raising pH >4 minimizes pepsin proteolytic activity, reducing gastric mucosal damage. * **Laxative**: Poorly absorbed magnesium ions create an osmotic gradient โ draws water into the intestinal lumen โ distends the bowel โ stimulates peristalsis. * **Rumen Alkalinizer**: Directly neutralizes excess lactic acid produced during grain overload โ increases rumen pH and alters rumen microflora activity.
Dosing by species
- Rumen overload syndrome ยท As above for cattle, but use โ th-ยผth the amount ยท PO ยท q6-12h
- Rumen overload syndrome ยท As above for cattle, but use โ th-ยผth the amount ยท PO ยท q6-12h
- Antacid ยท 5-15 mL PO once to twice daily ยท PO ยท q12-24h ยท Using Milk of Magnesia
- Rumen overload syndrome (Adults) ยท Up to 1 gram/kg (MgOH) mixed in 2-3 gallons of warm water and given PO per tube. May repeat (use smaller doses) at 612 hour intervals. If the rumen has been evacuated, do not exceed 225 grams initially. ยท PO ยท q6-12h ยท Dehydration and systemic acidosis must be concomitantly corrected.
- Rumen overload syndrome (Calves) ยท As above but use โ th-ยผth the amount ยท PO ยท q6-12h
- Adjunctive gastroduodenal ulcer therapy in foals ยท 15 mL 4 times a day ยท PO ยท q6h ยท Using Aluminum/magnesium hydroxide suspension
- Adjunctive treatment of hypomagnesemia with GI disease and severe hypocalcemia ยท 5-15 mL/per dog per 24 hours ยท PO ยท q24h ยท Using Milk of Magnesia
Routes of administration
Contraindications
- Renal disease (due to risk of hypermagnesemia)
- Gastric outlet obstruction (for aluminum-containing products)
- Use cautiously in patients on sodium- or potassium-restricted diets (some products contain significant amounts)
Adverse effects
- Diarrhea or frequent loose stools
- Hypermagnesemia (especially in patients with severe renal insufficiency)
- Constipation (if using combination products with aluminum)
- Hypophosphatemia (with chronic use of aluminum-containing products)
Drug interactions
- Quinidine ยท Increased absorption or pharmacologic effect may occur
- Sodium Polystyrene Sulfonate ยท May decrease the potassium-lowering effectiveness; may cause metabolic alkalosis in renal failure patients
- Sustained-release medications ยท May alter absorption by changing GI transit times when used at laxative dosages
- Sympathomimetic agents ยท Increased absorption or pharmacologic effect may occur
- Fluoroquinolones ยท Decreased absorption (chelation); separate doses by at least 2 hours
- Tetracyclines ยท Decreased absorption (chelation); separate doses by at least 2 hours
- Azole Antifungals (Ketoconazole, Itraconazole) ยท Decreased absorption due to increased gastric pH; separate doses by at least 2 hours
- Digoxin ยท Decreased absorption; separate doses by at least 2 hours
- Iron Salts ยท Decreased absorption; separate doses by at least 2 hours
- H-2 Antagonists ยท Decreased absorption; separate doses by at least 2 hours
- Thyroid Hormones ยท Decreased absorption; separate doses by at least 2 hours
- Corticosteroids ยท Decreased absorption; separate doses by at least 2 hours
Monitoring
- Electrolyte imbalances (Magnesium, Phosphorus, Calcium) with chronic or high-dose therapy
- Renal function
- Fecal consistency
Overdose
Chronic or acute overdose can lead to significant GI upset (severe diarrhea) and electrolyte imbalances (hypermagnesemia, especially in renal failure). Treat symptomatically with fluid therapy and correct electrolyte derangements.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturerโs current label.