What is a recommended over-the-counter (OTC) magnesium supplement and dosage?

Recommended OTC Magnesium Supplement and Dosing

For general supplementation, choose magnesium citrate or magnesium glycinate at 200-400 mg daily, as organic magnesium salts have superior bioavailability compared to inorganic forms like magnesium oxide. 1

Form Selection Based on Clinical Goal

For General Supplementation (No Constipation)

• Magnesium citrate or magnesium glycinate are preferred due to their higher bioavailability as organic salts 1
• Start with 200-400 mg elemental magnesium daily, which can be divided into twice-daily dosing for better tolerance 1
• The RDA is 320 mg/day for women and 420 mg/day for men 2
• Organic forms (citrate, glycinate, aspartate, lactate) are absorbed more efficiently than inorganic forms (oxide, hydroxide) 1, 3

For Constipation Management

• Magnesium citrate is the preferred choice due to its stronger osmotic effect that draws water into the intestines 1
• Alternative: Magnesium oxide 400-500 mg daily, which has been studied specifically for chronic constipation at doses up to 1.5 g/day 4, 1
• Milk of Magnesia (magnesium hydroxide) is another option: 30 mL (2,400 mg) at bedtime, costing approximately $1 or less daily 5, 6
• Start at lower doses and titrate upward based on response to minimize gastrointestinal side effects 4

Dosing Strategy

Starting Dose

• Begin with 200-400 mg elemental magnesium daily for general supplementation 1
• For constipation: Start with magnesium citrate 200-400 mg or magnesium oxide 400-500 mg daily 1, 2
• Divide doses throughout the day rather than taking a single large dose to improve absorption and reduce GI side effects 1

Dose Titration

• Gradually increase according to tolerance, as some patients may require up to 600 mg daily for therapeutic effect 1
• The upper limit from supplements alone is 350 mg/day according to older guidelines, though recent evidence suggests doses above this can be consumed without adverse events 7

Formulation Considerations

• Liquid or dissolvable formulations are better tolerated than pills and have fewer gastrointestinal side effects 1, 2
• Continuous-release formulations may improve absorption and reduce GI intolerance for long-term use 8

Critical Safety Precautions

Absolute Contraindications

• Avoid all magnesium supplements in patients with significant renal impairment (creatinine clearance <20 mL/min) due to life-threatening hypermagnesemia risk 4, 1, 5, 2
• Contraindicated in congestive heart failure patients 1

Monitoring and Side Effects

• Common side effects include diarrhea, bloating, and abdominal distension—these are dose-dependent and typically mild to moderate 4, 1
• Magnesium citrate causes more pronounced GI effects than other forms due to its stronger osmotic properties 1
• If diarrhea occurs, reduce the dose or switch to a different form 1

Clinical Decision Algorithm

1. Assess renal function first: If CrCl <20 mL/min, do not supplement 4, 1, 5

2. Identify primary goal:

   • Constipation present: Choose magnesium citrate 200-400 mg daily or milk of magnesia 30 mL at bedtime 1, 5
   • General supplementation only: Choose magnesium citrate or glycinate 200-400 mg daily 1

3. Select formulation: Prefer liquid/dissolvable over tablets for better tolerance 1, 2

4. Dosing schedule: Divide doses throughout the day (e.g., 200 mg twice daily) 1

5. Titrate based on response: Increase gradually if needed, monitoring for diarrhea 4, 1

Common Pitfalls to Avoid

• Do not use magnesium oxide for general supplementation—it has poor bioavailability and is primarily useful for constipation 1, 3
• Do not ignore renal function—this is the most critical safety consideration 4, 1, 5
• Do not start with high doses—begin low and titrate to avoid diarrhea that leads to discontinuation 4, 1
• Do not assume serum magnesium reflects total body stores—less than 1% of body magnesium is in the blood 2