What is the recommended dose of magnesium citrate for treating constipation?

Magnesium Citrate Dosing for Constipation

For adults with constipation, start with 6.5 to 10 fluid ounces (approximately 11-17 grams of magnesium citrate) as a single daily dose or divided doses, with a maximum of 10 fluid ounces in 24 hours, and ensure adequate hydration with a full 8-ounce glass of liquid with each dose. 1

Dosing by Age Group

Adults and children ≥12 years:

• Initial dose: 6.5 to 10 fl oz (approximately 11-17 grams magnesium citrate)
• Maximum: 10 fl oz in 24 hours
• Can be taken as single daily dose or divided doses 1

Children 6 to <12 years:

• Dose: 3 to 7 fl oz
• Maximum: 7 fl oz in 24 hours 1

Children 2 to <6 years:

• Dose: 2 to 3 fl oz in 24 hours
• Maximum: 3 fl oz in 24 hours 1

Children <2 years:

• Consult physician before use 1

Context: Magnesium Formulations for Chronic Constipation

While the FDA label provides specific dosing for magnesium citrate as an acute laxative, the 2023 AGA-ACG guidelines reference magnesium oxide (a related osmotic laxative) for chronic idiopathic constipation at 400-500 mg daily initially, with prior studies using 1,000-1,500 mg daily. 2 The guidelines note magnesium-based laxatives work through osmotic mechanisms and can be titrated based on symptom response with no clear maximum dose. 2

Critical Safety Considerations

Renal function assessment is mandatory before prescribing magnesium citrate:

• Contraindicated in patients with significant renal impairment due to risk of life-threatening hypermagnesemia 3
• Use with extreme caution in elderly patients, even with normal baseline renal function, as severe hypermagnesemia can develop in the setting of gastrointestinal disease (e.g., ischemic colitis, ileus) 4
• One case report documented a serum magnesium of 16.6 mg/dL (13.7 mEq/L) with sinus arrest after 34 grams of magnesium citrate in a 76-year-old woman without pre-existing renal dysfunction 4

Pregnancy considerations:

• Use with caution; lactulose is the only osmotic agent specifically studied in pregnancy 2

Potential Complications

Hypermagnesemia can cause:

• Paralytic ileus (documented in case reports with magnesium citrate and magnesium sulfate) 5
• Cardiac dysfunction including sinus arrest and junctional escape rhythm 4
• Hypotension and cardiovascular collapse 4
• Muscle weakness and flaccid paralysis 5

Common adverse effects:

• Abdominal cramping and discomfort 3
• Dehydration with reduced body weight and increased hemoglobin 6
• Postural hypotension in at-risk patients 6

Treatment Algorithm for Constipation

First-line therapy:

• Fiber supplements (14 g/1,000 kcal intake per day) with adequate hydration for mild constipation 3

Second-line options:

• Polyethylene glycol 17 g daily (preferred osmotic laxative with durable 6-month response) 2
• Magnesium citrate as alternative for patients who cannot tolerate PEG or as adjunct to fiber 3

Duration of therapy:

• For chronic constipation, treatment up to 4 weeks is established, though longer-term use is likely appropriate 3
• Reducing dosing frequency (e.g., three times daily to twice daily) while maintaining total daily dose does not affect laxative efficacy 7

Practical Administration

• Always administer with a full 8-ounce glass of water to minimize hypermagnesemia risk 1
• Shake liquid formulation well before use 1
• Monitor for adequate hydration throughout treatment 3
• Each fluid ounce contains 1.745 grams of magnesium citrate 1

Bioavailability Note

Magnesium citrate demonstrates superior bioavailability compared to magnesium oxide, with greater serum and salivary magnesium concentrations after both acute and chronic supplementation. 8 This suggests magnesium citrate may be more effective at lower doses than the magnesium oxide doses referenced in chronic constipation guidelines.