What is the recommended dose of milk of magnesia (magnesium hydroxide) for treating constipation?

Milk of Magnesium (Magnesium Hydroxide) Dosing for Constipation

For constipation treatment, the recommended initial dose of milk of magnesia (magnesium hydroxide) is 400-500 mg daily, which can be titrated based on symptom response and side effects. 1

Dosing Guidelines

• Magnesium oxide (milk of magnesia) is recommended as an osmotic laxative with a conditional recommendation based on very low quality evidence 1
• The recommended starting dose is 400-500 mg daily, though prior studies have used 1,000-1,500 mg daily 1
• Dose should be titrated based on symptom response and side effects 1
• No clear maximum dose has been established, but starting at a lower dose and increasing if necessary is suggested 1

Mechanism of Action and Efficacy

• Milk of magnesia works as an osmotic laxative, drawing water into the intestinal lumen to soften stool and promote bowel movements 1
• The American Gastroenterological Association suggests using magnesium oxide over no treatment for chronic idiopathic constipation 1

Safety Considerations and Contraindications

• Magnesium-based laxatives are contraindicated in patients with renal insufficiency due to risk of hypermagnesemia 1, 2
• Fatal cases of hypermagnesemia have been reported in patients taking magnesium hydroxide, particularly those with impaired kidney function 3
• Hypermagnesemia can occur even in patients with normal renal function in some circumstances 4, 3
• In patients with chronic kidney disease, lactulose is a safer alternative 2

Treatment Algorithm

1. First-line approach: Consider fiber supplements (particularly psyllium) for mild constipation before using osmotic laxatives 1
2. Second-line approach: For more significant constipation:
   • Start with milk of magnesia 400-500 mg daily if renal function is normal 1
   • Monitor response and adjust dose as needed 1
3. Alternative options if milk of magnesia is ineffective or contraindicated:
   • Polyethylene glycol (PEG) 17g daily (has stronger evidence and recommendation) 1
   • Lactulose 15g daily (safer in renal impairment) 2
   • Stimulant laxatives like bisacodyl (5-10 mg daily) or senna (8.6-17.2 mg daily) for short-term use or rescue therapy 1, 5

Monitoring and Follow-up

• Clinical trials with magnesium oxide were conducted for 4 weeks, though longer-term use is probably appropriate 1
• Regular monitoring of renal function is advisable with long-term use 3
• Watch for side effects including abdominal distension, loose stool, and nausea 1

Special Populations

• In children with chronic constipation, both milk of magnesia and polyethylene glycol have shown efficacy, though polyethylene glycol may have better compliance (95% vs 65%) 6
• In elderly patients or those with multiple medications, be particularly cautious about renal function before prescribing 3