No, Magnesium Citrate is NOT Milk of Magnesia
Magnesium citrate and milk of magnesia (magnesium hydroxide) are two distinct magnesium-based laxatives with different chemical compositions, mechanisms of action, and clinical properties.
Chemical Composition
- Magnesium citrate contains magnesium bound to citric acid, providing 1.745g per fluid ounce 1
- Milk of magnesia is magnesium hydroxide, containing 2,400mg per 30mL unit dose 2
- These are fundamentally different chemical compounds despite both containing magnesium 1, 2
Mechanism of Action
Magnesium Citrate
- Functions as an osmotic laxative by creating an osmotic gradient in the GI tract that draws water into the intestines 3, 4
- Has a stronger osmotic effect compared to other magnesium preparations, making it particularly effective for constipation 4
- When used in combination products (sodium picosulfate + magnesium citrate), the magnesium citrate component retains fluids in the colon 5
Milk of Magnesia (Magnesium Hydroxide)
- Also acts as an osmotic laxative but through a different chemical mechanism 2
- Has lower bioavailability as an inorganic magnesium salt compared to organic forms like magnesium citrate 4
Clinical Applications
When to Use Magnesium Citrate
- Bowel preparation before colonoscopy at doses of 300mL × 3 (900mL total) 4
- Constipation management as a second-line agent when first-line therapies are insufficient 4
- Preferred when a stronger osmotic effect is desired 4
When to Use Milk of Magnesia
- Chronic constipation as an inexpensive osmotic agent, typically 1 oz twice daily at approximately $1 or less per day 3
- Part of a gradual escalation strategy starting with fiber supplementation 3
Bioavailability Differences
- Organic magnesium salts (including citrate) have higher bioavailability compared to inorganic forms like magnesium hydroxide 4
- This bioavailability difference is clinically relevant when magnesium supplementation (not just laxative effect) is desired 4
Safety Considerations
Both Preparations Share Common Contraindications
- Avoid in significant renal impairment (creatinine clearance <20 mL/min) due to hypermagnesemia risk 4, 3
- Case reports document severe hypermagnesemia from both magnesium citrate and milk of magnesia in patients with renal dysfunction, including paralytic ileus 6 and near-fatal cardiac events 7
Magnesium Citrate-Specific Concerns
- Contraindicated in congestive heart failure due to hypermagnesemia risk 4
- Associated with higher risk of hyponatremia in patients ≥65 years (absolute risk increase 0.05%) 3
- May cause more pronounced GI effects (diarrhea, bloating) due to stronger osmotic properties 4
Clinical Decision-Making
For constipation management:
- Choose magnesium citrate when a stronger osmotic effect is needed 4
- Choose milk of magnesia for cost-effective daily maintenance therapy 3
For bowel preparation:
- Magnesium citrate is the appropriate choice, often combined with sodium picosulfate 3
For general magnesium supplementation:
- Magnesium citrate is preferred over milk of magnesia due to superior bioavailability 4
Common Pitfall to Avoid
Do not assume these products are interchangeable simply because both contain magnesium and have laxative effects. The chemical differences translate to different bioavailability profiles, osmotic strengths, and clinical applications 4, 1, 2.