Magnesium Citrate Dosing for Constipation
For acute constipation in adults, the standard dose of magnesium citrate is 8 ounces (240 mL) as a single daily dose, with a maximum of 10 fluid ounces in 24 hours. 1
FDA-Approved Dosing by Age
The FDA label provides clear age-based dosing guidelines 1:
- Adults and children ≥12 years: 6.5 to 10 fl oz (maximum 10 fl oz in 24 hours)
- Children 6 to <12 years: 3 to 7 fl oz (maximum 7 fl oz in 24 hours)
- Children 2 to <6 years: 2 to 3 fl oz (maximum 3 fl oz in 24 hours)
- Children <2 years: Consult physician
Always administer with a full 8-ounce glass of water with each dose to minimize hypermagnesemia risk. 1
Clinical Context and Treatment Algorithm
Acute vs. Chronic Constipation
The dosing differs significantly based on whether you're treating acute or chronic constipation:
For acute constipation: Use the full 8 oz (240 mL) dose as recommended by the National Comprehensive Cancer Network, which typically produces results within 30 minutes to 6 hours. 2, 3
For chronic constipation: Magnesium citrate is NOT the preferred formulation. The American Gastroenterological Association notes that only magnesium oxide (not citrate) has been studied in randomized controlled trials for chronic idiopathic constipation, at doses of 1.5 g/day for up to 4 weeks. 2, 3 Clinical practice typically uses lower doses of magnesium oxide (400-500 mg daily) for chronic management. 2, 3
Treatment Positioning
Magnesium citrate should be used as an alternative to polyethylene glycol for patients who cannot tolerate PEG, or as an adjunct to fiber supplementation. 2 The National Comprehensive Cancer Network recommends starting with 8 oz (240 mL) daily, titrated based on response with a goal of one non-forced bowel movement every 1-2 days. 2
Critical Safety Precautions
Absolute Contraindications
Do NOT use magnesium citrate in the following situations 2:
- Significant renal impairment (creatinine clearance <20-60 mL/min) - risk of fatal hypermagnesemia
- Suspected bowel obstruction - rule this out with physical exam and abdominal x-ray if clinically indicated
- Abdominal pain, nausea, or vomiting of unknown etiology
- Gastrointestinal diseases (ileus, ischemic colitis) - these patients are at increased risk for hypermagnesemia even with normal renal function 2, 4
High-Risk Populations Requiring Caution
Elderly patients are at particular risk for hypermagnesemia-induced complications even without pre-existing renal dysfunction. A case report documented severe hypermagnesemia (16.6 mg/dL) with cardiac arrest and ischemic colitis in a 76-year-old woman who received 34 g of magnesium citrate for ileus. 4 Another case series showed hypermagnesemia-induced paralytic ileus in elderly patients with normal or mildly impaired renal function. 5
Screen for medications that impair renal magnesium excretion before prescribing 3:
- ACE inhibitors
- NSAIDs
- Diuretics
Avoid rectal suppositories or enemas in patients with neutropenia or thrombocytopenia. 2
Monitoring Considerations
In patients taking daily magnesium oxide for chronic constipation, a retrospective study found that 16.6% developed high serum magnesium concentrations (≥2.5 mg/dL) and 5.2% developed hypermagnesemia (≥3.0 mg/dL). 6 Risk factors included CKD grade 4 and magnesium oxide dosage >1,000 mg/day. 6
Management of Persistent Constipation
If constipation persists after 4 weeks of magnesium citrate, reassess for impaction or obstruction, then consider 2:
- Adding stimulant laxatives: Bisacodyl 10-15 mg daily to three times daily
- Switching to alternative osmotic laxatives:
- Polyethylene glycol 17 g daily (preferred for durable 6-month response)
- Lactulose 30-60 mL twice to four times daily
- Sorbitol 30 mL every 2 hours × 3, then as needed
For opioid-induced constipation specifically, consider peripherally acting mu-opioid receptor antagonists (methylnaltrexone, naloxegol, naldemedine). 2
Common Pitfalls to Avoid
Rule out fecal impaction before initiating therapy, especially if diarrhea accompanies constipation (this indicates overflow around impaction). 2
Do not use osmotic laxatives in patients with known or suspected mechanical bowel obstruction. 2
Ensure adequate hydration during treatment - the dehydrating effect can lead to reduced bodyweight, increased hemoglobin levels, and postural hypotension in at-risk patients. 7, 8