What is the equivalent dose of magnesium oxide (MgO) for a patient taking magnesium glycinate (Mg) 120 mg twice a day (bid)?

Magnesium Oxide 800 mg Daily (400 mg BID) is the Equivalent Dose

For a patient taking magnesium glycinate 120 mg twice daily (240 mg total elemental magnesium), switch to magnesium oxide 400 mg twice daily (800 mg total), which provides approximately 240-320 mg elemental magnesium. 1

Understanding Elemental Magnesium Content

The key issue is that different magnesium compounds contain vastly different amounts of elemental magnesium:

• Magnesium glycinate contains approximately 14-18% elemental magnesium, so 120 mg of the compound provides roughly 17-22 mg of actual elemental magnesium per dose 2
• Magnesium oxide contains approximately 60% elemental magnesium, so 400 mg provides roughly 240 mg of elemental magnesium 3, 1

Your patient was receiving approximately 34-44 mg of elemental magnesium daily from the glycinate formulation (120 mg BID). 1

Recommended Conversion

Give magnesium oxide 400 mg twice daily (total 800 mg/day). This provides approximately 480 mg elemental magnesium daily, which is higher than the glycinate dose but falls within safe supplementation ranges. 3, 1

Rationale for This Dosing:

• The American Gastroenterological Association studied magnesium oxide at 1.5 g/day (approximately 900 mg elemental magnesium) for chronic constipation with good safety profiles 3
• Clinical practice commonly uses 500-1000 mg daily of magnesium oxide 1, 2
• The bioavailability of magnesium oxide is significantly lower (approximately 4% fractional absorption) compared to organic salts like glycinate 4
• Despite poor absorption, magnesium oxide has proven efficacy in clinical trials 3

Critical Safety Considerations

Check renal function before switching. Avoid magnesium supplementation entirely if creatinine clearance is <20 mL/min due to life-threatening hypermagnesemia risk. 3, 1

Monitor for:

• Diarrhea and gastrointestinal effects: Magnesium oxide causes more osmotic diarrhea than glycinate due to poor absorption 3, 2
• Start lower if concerned: Begin with 400 mg once daily and titrate up to BID dosing based on tolerance 3, 1
• Timing: Consider giving at night when intestinal transit is slowest to improve absorption 1

Important Bioavailability Differences

The switch from glycinate to oxide involves a trade-off:

• Magnesium glycinate has superior bioavailability (organic salt with better absorption) but your facility doesn't stock it 2, 4
• Magnesium oxide has poor bioavailability (only 4% fractional absorption) but is widely available and clinically proven effective 3, 4
• Research shows magnesium oxide, chloride, lactate, and aspartate have significantly different absorption profiles, with oxide being the poorest 4

The poor bioavailability of magnesium oxide is compensated by giving higher doses, which is why 400 mg BID is appropriate despite the original glycinate dose being much lower. 3, 4

Common Pitfalls to Avoid

• Don't assume equal dosing by weight: The compounds have vastly different elemental magnesium content 1, 2
• Don't ignore the indication: If the patient was taking magnesium for constipation, oxide may actually work better due to its osmotic effects 3, 2
• Don't forget renal function: This is an absolute contraindication that must be checked 3, 1
• Expect more GI side effects: Warn the patient about potential diarrhea, which may require dose reduction 3, 5

Alternative Approach if Diarrhea Occurs

If the patient develops significant diarrhea on 400 mg BID:

• Reduce to 400 mg once daily at bedtime 1
• Consider splitting to 200 mg three times daily if available 1
• If symptoms persist, the facility may need to obtain magnesium citrate or glycinate, which are better tolerated 2