Magnesium Citrate Dosing for Acute Constipation
For acute constipation in adults, administer 6.5 to 10 fluid ounces (approximately 195-300 mL) of magnesium citrate as a single dose or divided doses, with a maximum of 10 fluid ounces in 24 hours. 1
FDA-Approved Dosing by Age
The FDA label provides clear age-stratified dosing for magnesium citrate oral solution (1.745g magnesium citrate per fluid ounce): 1
- Adults and children ≥12 years: 6.5 to 10 fl oz (maximum 10 fl oz in 24 hours) 1
- Children 6 to <12 years: 3 to 7 fl oz (maximum 7 fl oz in 24 hours) 1
- Children 2 to <6 years: 2 to 3 fl oz (maximum 3 fl oz in 24 hours) 1
- Children <2 years: Consult a physician 1
Critical Implementation Points
Always ensure patients drink a full 8-ounce glass of liquid with each dose to prevent dehydration and minimize risk of hypermagnesemia. 1
Mechanism and Expected Onset
Magnesium citrate works through osmotic mechanisms, drawing water into the intestinal lumen to soften stool and stimulate bowel movements, typically producing results within 30 minutes to 6 hours. 2, 3
Absolute Contraindications
Avoid magnesium citrate in patients with significant renal impairment (creatinine clearance <20-60 mL/min) due to risk of fatal hypermagnesemia. 4, 2 Even patients with normal renal function have developed fatal hypermagnesemia when magnesium-based laxatives are retained in the gut, serving as a continuous absorption reservoir. 5
Comparison to Chronic Constipation Management
While the question asks about acute constipation, it's important to note that chronic constipation management differs significantly:
- For chronic idiopathic constipation, magnesium oxide (not citrate) has been studied at 1.5 g/day for up to 4 weeks 4
- Clinical practice often uses lower doses (400-500 mg daily) for chronic management 2
- The bioavailability and clinical efficacy of magnesium citrate specifically for chronic constipation are unknown, as only magnesium oxide has been evaluated in randomized controlled trials 4
Common Pitfalls to Avoid
Do not use magnesium citrate repeatedly for chronic constipation without medical supervision, as this increases risk of hypermagnesemia, particularly in patients with unrecognized renal dysfunction or conditions causing gut stasis. 5 The case literature documents fatal outcomes even with normal baseline renal function when magnesium-based laxatives accumulate. 5
Screen for medications that impair renal magnesium excretion (ACE inhibitors, NSAIDs, diuretics) before prescribing, as these increase hypermagnesemia risk. 4
Efficacy Data
For bowel preparation procedures, magnesium citrate 250-300 mL has demonstrated 75-94% satisfactory cleansing rates, superior to single-dose polyethylene glycol regimens. 4, 6 This supports its efficacy for acute bowel evacuation at the FDA-recommended doses.