Magnesium citrate

How it works

Poorly absorbed magnesium ions exert an osmotic effect that draws water into the bowel lumen, increasing intraluminal volume and stimulating colonic motility to produce evacuation.

Prescribing in practice

• It is contraindicated in intestinal obstruction and should be avoided or used with great caution in significant renal impairment, where reduced excretion risks hypermagnesaemia.
• Ensure adequate hydration, as the purgative fluid losses can precipitate dehydration and electrolyte disturbance, particularly in elderly patients.
• Magnesium can chelate or reduce absorption of certain co-administered drugs, so the timing of other oral medicines may need separation.

Monitoring

Monitor renal function and consider serum magnesium in patients with impaired kidney function or when repeated doses are given.

Counselling the patient

• Drink plenty of clear fluid while the medicine works to avoid becoming dehydrated.
• Tell your prescriber if you have kidney problems before taking it.
• Effects come on quickly, so remain close to a toilet.

Evidence & guidelines

Magnesium salts are long-established osmotic laxatives, and current prescribing references caution against their use in renal impairment because of hypermagnesaemia risk.