Slow Magnesium vs. Magnesium Chloride
"Slow magnesium" is not the same as magnesium chloride—it refers to a slow-release (enteric-coated) formulation of magnesium chloride, which is a specific delivery method of the magnesium chloride salt.
Understanding the Terminology
- Magnesium chloride is a specific chemical salt form of magnesium that provides elemental magnesium 1
- "Slow Mag" or "slow magnesium" is a brand name for enteric-coated, slow-release magnesium chloride tablets that delay dissolution to improve gastrointestinal tolerance and absorption 2
- The term "slow" refers to the pharmaceutical formulation (controlled-release), not a different type of magnesium salt 3
Key Differences in Formulation
Standard Magnesium Chloride
- Releases magnesium rapidly, typically within 1 hour of dissolution 4
- Higher bioavailability compared to magnesium oxide (fractional absorption significantly better than the 4% seen with magnesium oxide) 1
- May cause more gastrointestinal side effects with immediate release 2
Slow-Release Magnesium Chloride
- Provides continuous magnesium release over 6 hours throughout the gastrointestinal tract 4
- Designed to decrease risk of gastrointestinal side effects, allowing long-term supplementation 4
- Shows comparable 24-hour bioavailability to immediate-release forms but with different time-concentration profiles 3
- Associated with significantly fewer side effects (20%) compared to other oral agents in clinical trials 2
Clinical Implications
When Formulation Matters
- For cardiac emergencies: IV magnesium sulfate 1-2 g bolus is recommended, not oral formulations 5
- For chronic supplementation: Slow-release formulations may improve adherence due to better gastrointestinal tolerance 4, 2
- For mild hypomagnesemia: Magnesium oxide 12-24 mmol daily is the preferred first-line oral supplement despite lower bioavailability, as it contains more elemental magnesium per dose 6
Bioavailability Considerations
- Magnesium chloride (both immediate and slow-release) has significantly higher bioavailability than magnesium oxide 1
- Organic salts (aspartate, citrate, lactate) have higher bioavailability than magnesium oxide or hydroxide 5, 6
- Slow-release magnesium chloride achieves 76% cumulative urinary magnesium at 0-5 hours, 89% at 0-10 hours, and 87% at 0-24 hours compared to standard formulations 4
Common Pitfalls to Avoid
- Don't confuse the brand name with the chemical compound: "Slow Mag" is enteric-coated magnesium chloride, not a different magnesium salt 2
- Don't assume all magnesium chloride is slow-release: Standard magnesium chloride solutions and tablets release immediately 3
- Don't use magnesium chloride routinely in parenteral nutrition: It increases anion gap and metabolic acidosis risk; magnesium sulfate is preferred 5