What is in Miralax and is Magnesium Hydroxide Equivalent?
Miralax contains polyethylene glycol 3350 (PEG 3350) as its active ingredient, and it is NOT equivalent to magnesium hydroxide—these are fundamentally different laxatives with distinct mechanisms of action, side effect profiles, and clinical applications. 1
Active Ingredient in Miralax
- Miralax contains PEG 3350, which is an inert polymer of ethylene glycol that works as an osmotic laxative by sequestering fluid in the bowel 1
- PEG 3350 is a long-chain polymer of ethylene oxide that draws water into the intestinal lumen, softening stool and increasing bowel movements 1, 2
- The standard dose is 17 g once daily mixed in 8 ounces of liquid 1, 3
- PEG 3350 is minimally absorbed systemically and does not cause significant net gain or loss of sodium and potassium, contributing to its favorable safety profile 4
Magnesium Hydroxide (Milk of Magnesia)
- Magnesium hydroxide is a different osmotic laxative that increases water in the large bowel by drawing fluid from the body into the bowel 1
- Magnesium salts are useful where rapid bowel evacuation is required 1
- Magnesium-based preparations are contraindicated in patients with congestive heart failure, hypermagnesemia, and severe renal impairment due to their hyperosmolar nature 1
- Magnesium preparations can cause transient electrolyte imbalances, particularly in patients with compromised renal function 1
Key Differences Between PEG 3350 and Magnesium Hydroxide
Mechanism of Action
- PEG 3350 retains fluid that was administered with it (sequestering mechanism), while magnesium hydroxide draws fluid from the body into the bowel 1
- This fundamental difference makes them non-interchangeable 1
Safety Profile
- PEG 3350 has superior safety for long-term use with no clinically significant changes in electrolytes, even after 12 months of continuous use 5, 6
- Magnesium hydroxide carries risk of hypermagnesemia, particularly in elderly patients or those with renal impairment 1
- PEG 3350 can be used safely in patients with heart failure and renal disease, where magnesium preparations are contraindicated 1, 4
Duration of Use
- PEG 3350 is proven safe and effective for chronic use up to 12 months with no evidence of tachyphylaxis 5, 6
- Magnesium hydroxide is typically recommended for short-term or occasional use due to electrolyte concerns 1
Efficacy for Chronic Constipation
- PEG 3350 demonstrates 52% treatment success rate versus 11% for placebo in chronic constipation over 6 months 5
- PEG 3350 is recommended by the American Gastroenterological Association as a first-line agent for chronic constipation with strong recommendation and moderate certainty of evidence 1, 2
Common Pitfalls and Caveats
- Do not assume osmotic laxatives are interchangeable—the electrolyte and safety profiles differ significantly 1
- Avoid magnesium-based laxatives in elderly patients, those with renal disease, or heart failure—use PEG 3350 instead 1
- PEG 3350 requires adequate fluid intake (8 ounces per dose) for optimal efficacy 3
- Common side effects of PEG 3350 include diarrhea, flatulence, nausea, and abdominal bloating, but these are generally mild 4, 5
- One rare case report exists of PEG 3350 contamination with ethylene glycol, but this is exceptionally uncommon and not a contraindication to use 7
Clinical Bottom Line
Choose PEG 3350 (Miralax) over magnesium hydroxide for most patients, especially for chronic constipation, elderly patients, those with renal disease, or heart failure, as it has superior safety, proven long-term efficacy, and fewer contraindications. 1, 5, 6