Magnesium citrate

Also called mag citrate.

Dr Chad OkayFounder, Suna Health

4 min read. Updated .

Magnesium citrate is magnesium bound to citric acid. It is moderately well-absorbed, but its main use is its osmotic effect in the colon: it pulls water in and softens stool. Trials of magnesium salts for constipation typically used 200 to 400 mg of elemental magnesium in the evening. Worth knowing: the 2023 AGA/ACG constipation guideline puts macrogol first, and its magnesium evidence is for the oxide, not the citrate. Ask a pharmacist before starting any of them.

How it works

Magnesium citrate is moderately well-absorbed in the small intestine, intestinal magnesium absorption varies roughly from 35% to 70% depending on dose and status, with citrate showing higher bioavailability than oxide in head-to-head comparisons (Walker 2003). For laxation this cuts the other way: it is the unabsorbed fraction that reaches the colon and pulls water osmotically into the lumen, which is part of why the constipation trials behind the 2023 AGA/ACG guideline used the cheaper, less-absorbed oxide. This softens the stool and stimulates colonic motility. Onset for the laxative effect at supplement doses ranges from about 30 minutes to 6 hours; evening use typically produces a morning bowel movement.

When to use it

  • Slow-transit constipation that has not responded to fibre + water + movement, though the 2023 AGA/ACG guideline's strong recommendation is macrogol, with magnesium (the oxide, conditionally) behind it.
  • GLP-1-induced constipation (Ozempic, Wegovy, Mounjaro), as one of the osmotic options.
  • Travel constipation.
  • Daily use for chronic constipation only under a doctor's guidance.
  • NOT for diarrhoea or loose stools (will make worse).
  • Not the best form for sleep or anxiety; glycinate is the form usually preferred for those, though evidence is limited.

Magnesium citrate vs glycinate

  • Citrate: pulls water into colon, softens stool, mild laxative.
  • Glycinate: gentle on gut, no laxative effect, often preferred for sleep and anxiety (though the trial evidence specific to glycinate is limited).
  • Both well-absorbed.
  • Citrate is cheaper.
  • Some people use different forms for different jobs. If you are thinking of combining magnesium supplements, ask a pharmacist, and keep the combined total within the supplemental guidance levels (UK 400 mg a day, US 350 mg).

What the trials and references describe

  • Constipation trials of magnesium salts typically used 200 to 400 mg of elemental magnesium a day, taken in the evening. That is a description of the studies, not a dose to copy. A pharmacist can advise on a starting amount for you.
  • Supplemental magnesium has guidance ceilings: 400 mg a day in the UK (EVM) and 350 mg in the US, with diarrhoea as the effect that sets the limit.
  • The laxative effect typically appears within 30 minutes to 6 hours, so evening use usually means a morning bowel movement.
  • Diarrhoea means the amount is too high for you.

What to watch

  • Kidney disease: cannot clear excess magnesium safely. Talk to a doctor before use.
  • On other meds: space out from antibiotics and bisphosphonates by 2 hours, and from thyroid meds (levothyroxine) by 4 hours.
  • Higher supplement doses commonly cause diarrhoea; the upper level for supplemental magnesium is 350 mg/day, with loose stools as the limiting effect, and electrolyte loss possible.
  • If you need it daily for more than 4 to 6 weeks, see a doctor: chronic constipation deserves investigation, not just laxatives.
  • See a doctor promptly, at any age, for blood in the stool, unintentional weight loss, or a change in bowel habit that has not settled. NICE now recommends a simple stool test (FIT) for an unexplained change in bowel habit with no minimum duration.

Common questions

Will I get dependent on it?
Magnesium citrate does not cause laxative dependence the way stimulant laxatives (senna, bisacodyl) can. The osmotic effect is mechanical, not nerve-based. Long-term daily use is reasonable if needed.
How fast does it work?
Roughly 30 minutes to 6 hours, depending on dose. An evening supplement dose typically produces a morning bowel movement. High-dose 'liquid' magnesium citrate (used for bowel prep) is at the faster end and far more aggressive. That is a different product for a different job.
Can I take it with food?
Yes. Food slightly reduces absorption but improves tolerance, so many people take it with or shortly after their evening meal, with water.
Does it interact with my GLP-1?
No direct interaction. Osmotic laxatives are a standard option for GLP-1 constipation; guidelines usually put polyethylene glycol (PEG/Miralax) first, with magnesium as an alternative. Take it at bedtime; doesn't affect the weekly GLP-1 dose.

Sources

Written by Dr Chad Okay. Contributor: Dr Parisah Hussain, MBChB, BSc (Hons), DRCOG, MRCGP. Last updated 8 September 2026. From Digests.

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