Magnesium Hydroxide
Magnesium hydroxide
Also known as: Milk of Magnesia · Magnalax · Carmilax · Polymag · Rumen Bolus · Instamag · Polyox · Laxade · Maalox · Phillips' Chewable · Magnesium/Aluminum Antacids
Reviewed by the VetSheet Veterinary TeamUpdated Apr 5, 2026Evidence-based veterinary reference
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Dosing by species
🌎 NA — North America🌍 EU — Europe
Dogs
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Adjunctive treatment of hypomagnesemia with GI disease and severe hypocalcemia | 5-15 mL/per dog per 24 hours | PO | q24h | — | 🌎 NA |
| Adjunctive therapy for gastric ulcers | 2-10 mL PO q2-4h | PO | q2-4h | — | 🌎 NA |
| Antacid | 5-30 mL PO once to twice daily | PO | q12-24h | — | 🌎 NA |
- Adjunctive treatment of hypomagnesemia with GI disease and severe hypocalcemia: Using Milk of Magnesia
- Adjunctive therapy for gastric ulcers: Using Aluminum hydroxide suspension or aluminum hydroxide/magnesium hydroxide suspension
- Antacid: Using Milk of Magnesia
Cats
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Antacid | 5-15 mL PO once to twice daily | PO | q12-24h | — | 🌎 NA |
- Antacid: Using Milk of Magnesia
Horses
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Adjunctive gastroduodenal ulcer therapy in foals | 15 mL 4 times a day | PO | q6h | — | 🌎 NA |
- Adjunctive gastroduodenal ulcer therapy in foals: Using Aluminum/magnesium hydroxide suspension
Cattle
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Rumen overload syndrome (Adults) | 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. | PO | q6-12h | — | 🌎 NA |
| Rumen overload syndrome (Calves) | As above but use ⅛th-¼th the amount | PO | q6-12h | — | 🌎 NA |
- Rumen overload syndrome (Adults): Dehydration and systemic acidosis must be concomitantly corrected.
Sheep
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Rumen overload syndrome | As above for cattle, but use ⅛th-¼th the amount | PO | q6-12h | — | 🌎 NA |
Goats
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Rumen overload syndrome | As above for cattle, but use ⅛th-¼th the amount | PO | q6-12h | — | 🌎 NA |
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Overview
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 of action
Acts locally in the gastrointestinal tract:
- Antacid: Neutralizes gastric hydrochloric acid (HCl) → raises gastric pH. Raising pH >4 minimizes pepsin proteolytic activity, reducing gastric mucosal damage.
- Laxative: Poorly absorbed magnesium ions create an osmotic gradient → draws water into the intestinal lumen → distends the bowel → stimulates peristalsis.
- Rumen Alkalinizer: Directly neutralizes excess lactic acid produced during grain overload → increases rumen pH and alters rumen microflora activity.
Safety & warnings
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)
Precautions
Magnesium-containing antacids are contraindicated in patients with renal disease. Oral magnesium hydroxide should only be used clinically in ruminants with documented rumen acidosis and should not be used for treatment of other suspected rumen disorders or hypomagnesemia. Chronic use may lead to electrolyte abnormalities.
Drug interactions
Increased absorption or pharmacologic effect may occur
May decrease the potassium-lowering effectiveness; may cause metabolic alkalosis in renal failure patients
May alter absorption by changing GI transit times when used at laxative dosages
Increased absorption or pharmacologic effect may occur
Decreased absorption (chelation); separate doses by at least 2 hours
Decreased absorption (chelation); separate doses by at least 2 hours
Decreased absorption due to increased gastric pH; separate doses by at least 2 hours
Decreased absorption; separate doses by at least 2 hours
Decreased absorption; separate doses by at least 2 hours
Decreased absorption; separate doses by at least 2 hours
Decreased absorption; separate doses by at least 2 hours
Decreased absorption; separate doses by at least 2 hours
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
Pharmacokinetics
Absorption
Generally relatively non-absorbable, but up to 20% of an oral dose of magnesium can be absorbed.
Elimination
Absorbed magnesium is excreted by the kidneys.
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.
Available products
Formulations
- Oral Boluses
- Oral Powder
- Oral Liquid (Milk of Magnesia)
- Chewable Tablets
- Oral Suspension
Veterinary
- Oral Boluses 17.9-27 grams (Magnalax, Carmilax, Polymag, Rumen Bolus, Instamag, Polyox II, Laxade)
- Oral Powder 350-361 grams/lb (Carmilax Powder, Magnalax, Polyox, Laxade)
- Milk of Magnesia 80 mg/mL in gallons
Human-labeled
- Chewable Tablets: 311 mg & 400 mg (Phillips' Chewable, Pedia-Lax)
- Oral Liquid (Milk of Magnesia): 400 mg/5 mL
- Liquid concentrate: 800 mg/5 mL & 1,200 mg/5 mL
- Aluminum Hydroxide and Magnesium Hydroxide Suspension (e.g., Maalox)
Regulatory status
No regulatory data for: 🇺🇸 US · 🇪🇺 EU · 🇬🇧 UK · 🇭🇰 HK · 🇹🇼 TW · 🇯🇵 JP · 🇰🇷 KR · 🇦🇺 AU
Storage & stability
Store at room temperature in tight containers.
Client information
- Oral magnesium hydroxide products (like Milk of Magnesia) are available over-the-counter (OTC), but do not give on a regular basis without veterinary supervision.
- May cause diarrhea. Ensure your pet has access to plenty of fresh water.
- Separate this medication from other oral drugs by at least 2 hours to prevent absorption issues.
- Contact your veterinarian if your pet becomes lethargic or if diarrhea persists.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturer’s current label.