Is there a benefit to using polyethylene glycol (Miralax) and magnesium hydroxide (Milk of Magnesia) together for constipation?

Using Miralax and Milk of Magnesia Together for Constipation

Combining polyethylene glycol (Miralax) and magnesium hydroxide (Milk of Magnesia) together for constipation offers no proven additional benefit over using either agent alone and may increase the risk of side effects.

Mechanism of Action and Evidence

Both medications work as osmotic laxatives but through different mechanisms:

• Polyethylene glycol (PEG/Miralax): Sequesters fluid in the bowel by drawing water into the intestinal lumen 1
• Magnesium hydroxide (Milk of Magnesia): Draws fluid from the body into the bowel and has some stimulant properties 1

The available guidelines do not recommend combining these agents as standard practice. Instead, they suggest a stepwise approach:

1. Start with a single osmotic laxative (PEG or magnesium salts)
2. Add a stimulant laxative if osmotic therapy alone is inadequate 1

Comparative Efficacy

When used individually, the evidence shows:

• PEG vs. Milk of Magnesia: Meta-analysis of 3 studies (211 participants) showed PEG produced slightly more stools per week than Milk of Magnesia (MD 0.69,95% CI 0.48 to 0.89), though this difference may not be clinically significant 2

• PEG vs. Placebo: PEG significantly increases stool frequency (MD 2.61 stools per week, 95% CI 1.15 to 4.08) 2

• PEG vs. Lactulose: PEG is more effective than lactulose, with patients on PEG requiring fewer additional laxative therapies 2

Safety Considerations

Combining these medications may increase the risk of:

• Diarrhea
• Abdominal cramping
• Electrolyte disturbances
• Dehydration

Important Cautions:

• Magnesium-based preparations should be avoided in patients with chronic kidney disease due to the risk of magnesium toxicity 1
• Monitoring is particularly important when using multiple osmotic agents together

Clinical Application

For constipation management, a more appropriate approach would be:

1. First-line: PEG alone (polyethylene glycol) is considered first-line therapy for functional constipation in both adults and children 3

2. If inadequate response: Consider adding a stimulant laxative rather than another osmotic agent 1

3. Special circumstances: In postoperative settings, Milk of Magnesia has been shown to reduce hospital length of stay by 14.1% following colorectal surgery 4, but this does not support combination use with PEG

Conclusion

While both medications are effective individually, there is no evidence supporting the combined use of Miralax and Milk of Magnesia. Using them together may increase the risk of adverse effects without providing additional therapeutic benefit. The preferred approach is to optimize the dose of a single osmotic agent before considering combination therapy with a different class of laxative.