Understanding "8% Milk of Magnesia"
Your patient is likely referring to the standard concentration of magnesium hydroxide suspension (milk of magnesia), which is approximately 8% w/v (400 mg/5 mL or 2,400 mg/30 mL), the typical over-the-counter formulation available in the United States 1.
What This Means Clinically
The "8%" designation refers to the concentration of magnesium hydroxide in the suspension:
- Standard OTC milk of magnesia contains 400 mg magnesium hydroxide per 5 mL
- This translates to approximately 8% weight/volume concentration
- A typical adult dose is 30-60 mL (2,400-4,800 mg magnesium hydroxide) 1
Appropriate Dosing
For constipation in adults:
- Standard dose: 30-60 mL once daily, preferably at bedtime 1
- This provides 2,400-4,800 mg of magnesium hydroxide
- Should be taken with a full 8 oz glass of water 1
Important clinical context from guidelines:
- While clinical trials studied magnesium oxide (MgO) at 1.5 g/day, lower doses of 500 mg to 1 g/day are commonly used in clinical practice 2
- The AGA/ACG 2023 guidelines suggest starting at a lower dose and increasing as necessary 2
- Treatment duration in trials was 4 weeks, though longer-term use is appropriate 2
Critical Safety Considerations
Absolute contraindications:
- Avoid in patients with creatinine clearance <20 mL/min due to hypermagnesemia risk 2
- Kidney disease requires physician consultation before use 1
Warning signs to monitor:
- Hypermagnesemia can occur even with normal renal function if bowel obstruction is present 3
- Stop use if no bowel movement after use or rectal bleeding occurs 1
- Do not use for more than 1 week without physician evaluation 1
Common Pitfall
The confusion likely stems from patients or pharmacies referring to the concentration percentage rather than the more familiar volume-based dosing. There is no separate "8%" formulation—this is simply the standard milk of magnesia concentration that has been available for decades.