Milk of Magnesia Dosing for Adult Constipation
For an adult with no significant medical history, the standard dose of milk of magnesia (magnesium hydroxide) is 30-60 mL (2,400-4,800 mg) taken once daily, preferably at bedtime, with a full 8-ounce glass of water. 1
Standard Dosing Protocol
Adults and children over 12 years:
- Initial dose: 30 mL (one unit dose) taken at bedtime 1
- Maximum dose: 60 mL (two unit doses) per day 1
- Administration: Shake well before use and drink a full glass (8 oz) of liquid with each dose 1
- Timing: Preferably at bedtime when intestinal transit is slowest to optimize the osmotic effect 2
The FDA-approved labeling specifies not to exceed the maximum recommended daily dose in a 24-hour period, and the dose may be taken once daily, in divided doses, or as directed by a physician 1.
Mechanism and Expected Response
Milk of magnesia works through an osmotic mechanism, drawing water into the intestinal lumen to soften stool and stimulate bowel movements 2. The goal is to achieve one non-forced bowel movement every 1-2 days 2.
Critical Contraindications and Safety Precautions
Absolute contraindications:
- Renal impairment: Creatinine clearance <20 mL/min due to risk of fatal hypermagnesemia 3, 2, 4, 5
- Suspected bowel obstruction 2
- Abdominal pain, nausea, or vomiting of unknown etiology 2
Use with extreme caution in:
- Older adults due to increased risk of hypermagnesemia 3, 4
- Patients with gastrointestinal diseases (ileus, ischemic colitis) who have increased risk even with normal renal function 2
- Non-ambulatory patients with low fluid intake (risk of mechanical obstruction) 3
Treatment Algorithm for Persistent Constipation
If constipation persists after 4 weeks:
- Reassess for fecal impaction or obstruction 2
- Consider adding a stimulant laxative (bisacodyl 10-15 mg daily) 2
- Switch to alternative osmotic laxative: polyethylene glycol 17 g daily, lactulose 30-60 mL twice to four times daily, or sorbitol 2
Common Pitfalls to Avoid
- Never use in patients with significant renal dysfunction: Even modest renal impairment can lead to life-threatening hypermagnesemia, as demonstrated by case reports of severe bradycardia, asystole, and death in patients with end-stage renal disease who used milk of magnesia 5, 6
- Ensure adequate hydration: Failure to drink sufficient fluids reduces efficacy and increases risk of complications 2
- Rule out bowel obstruction first: Use physical exam and abdominal x-ray if clinically indicated before initiating therapy 2
- Avoid in patients with swallowing disorders: Risk of aspiration, particularly in bed-bound patients 3
- Most magnesium salts are poorly absorbed and may worsen diarrhea: This can create a vicious cycle of magnesium depletion through increased gastrointestinal losses 4
Comparative Efficacy
Polyethylene glycol (PEG) and milk of magnesia demonstrate equal efficacy for chronic constipation in long-term treatment, though PEG shows better patient acceptance (95% compliance vs 65% for milk of magnesia) 7. PEG 17 g daily is the preferred osmotic laxative with durable 6-month response, and magnesium-based laxatives can be used as alternatives for patients who cannot tolerate PEG 2.