What is the typical dosage of magnesium citrate (Mag citrate) for constipation?

Magnesium Citrate Tablet Dosage for Constipation

For adults with constipation, start magnesium citrate at 400-500 mg daily and titrate up to 1,000-1,500 mg daily based on response, though note that liquid formulations (6.5-10 fl oz for adults) are more commonly available and FDA-approved than tablets. 1, 2

Standard Dosing Recommendations

Liquid Formulation (Most Common)

• Adults and children ≥12 years: 6.5 to 10 fl oz (maximum 10 fl oz in 24 hours), taken as a single daily dose or divided doses 2
• Children 6 to <12 years: 3 to 7 fl oz (maximum 7 fl oz in 24 hours) 2
• Children 2 to <6 years: 2 to 3 fl oz in 24 hours (maximum 3 fl oz in 24 hours) 2
• Each fluid ounce contains 1.745 g of magnesium citrate 2
• Always drink a full glass (8 ounces) of liquid with each dose 2

Tablet/Powder Formulation Guidance

• The American Gastroenterological Association recommends starting with 400-500 mg daily for chronic idiopathic constipation, with prior studies demonstrating efficacy at 1,000-1,500 mg daily 1
• Magnesium oxide at 1.5 g/day has shown significant efficacy in clinical trials, suggesting comparable osmotic laxatives like magnesium citrate would be effective at similar doses 1
• Dosing can be once daily at bedtime or divided into twice-daily administration 3

Treatment Algorithm

Step 1: Begin with dietary modifications and fiber supplementation with adequate hydration 1

Step 2: If insufficient response, add magnesium citrate at lower doses (400-500 mg daily or 30 mL liquid at bedtime) 3, 1

Step 3: Titrate dose upward based on symptom response—no clear maximum dose exists, but typical effective range is 1,000-1,500 mg daily 1

Step 4: If constipation persists after 4 weeks, consider adding a stimulant laxative (bisacodyl 10-15 mg 2-3 times daily) or switching to polyethylene glycol 4, 1

Step 5: For refractory cases, consider alternative osmotic agents or prokinetic agents if gastroparesis is suspected 4

Mechanism and Efficacy

• Magnesium citrate works as an osmotic laxative by drawing water into the intestinal lumen, softening stool and stimulating bowel movements 1
• Clinical trials demonstrate magnesium oxide (a comparable osmotic agent) significantly increases complete spontaneous bowel movements per week, total spontaneous bowel movements, and quality of life scores 1
• The European Society for Medical Oncology strongly endorses osmotic laxatives including magnesium salts as preferred agents for chronic constipation 3

Critical Safety Precautions

Absolute Contraindication

• Do not use in patients with significant renal impairment (creatinine clearance <20 mL/min) due to risk of fatal hypermagnesemia 3, 1

High-Risk Populations Requiring Caution

• Elderly patients: Increased risk of electrolyte disturbances and hypermagnesemia 3
• Patients with any degree of renal dysfunction: Even those with normal baseline renal function can develop severe hypermagnesemia if constipation causes retention of magnesium in the gut, creating a reservoir for continuous absorption 5, 6
• Pregnant women: Use only under medical supervision; lactulose is the only osmotic agent specifically studied in pregnancy 1

Warning Signs of Hypermagnesemia

• Lethargy, hypotension, cardiac conduction abnormalities (sinus arrest, junctional rhythm) 6
• Severe hypermagnesemia can be fatal even with prompt renal replacement therapy 5
• Excessive doses can lead to hypermagnesemia even in patients with normal renal function, particularly in elderly patients with gastrointestinal diseases 3, 6

Important Clinical Pearls

• Ensure adequate hydration during treatment to minimize hypermagnesemia risk 1
• Magnesium citrate can be used as an alternative to polyethylene glycol for patients who cannot tolerate PEG, or as an adjunct to fiber supplementation 1
• If constipation presents with diarrhea, suspect fecal impaction with overflow—this requires different management (glycerine suppositories or manual disimpaction) 4, 3
• Treatment duration of up to 4 weeks has been studied, though longer-term use is likely appropriate for chronic constipation 1
• The number of daily doses can be reduced (from three to two, or two to one) without affecting laxative efficacy, as long as the total daily dose remains constant 7