Last updated: October 2026
Magnesium from a supplement is mostly absorbed or passed within a day or two. Absorption happens over several hours, your kidneys clear any excess within about 24 hours, and unabsorbed magnesium leaves in your stools. But magnesium stored in bones and muscles builds up over weeks, which is why daily use matters more than timing.
Below: what happens to a dose hour by hour, differences between forms, whether magnesium builds up, how long it takes to work, and how to clear excess magnesium if you’ve taken too much.
What happens to a dose of magnesium
| Stage | What happens | Roughly when |
|---|---|---|
| Absorption | About 30 to 40% of the magnesium you take in is absorbed, mostly in the small intestine. Absorption is higher when you’re low and lower with large doses. | Over several hours |
| In the blood | Less than 1% of your body’s magnesium is in your blood at any time; it’s kept within a tight range. | Constantly balanced |
| Into tissues | Most magnesium is stored in bones (about 60%) and in muscles and soft tissues. | Builds up over weeks |
| Kidneys | The kidneys filter magnesium and reabsorb about 95%, passing out what you don’t need in urine. | Excess cleared within about 24 hours |
| Unabsorbed magnesium | Leaves in your stools, and can loosen them. | Usually within 1 to 3 days |
An adult body holds roughly 24g of magnesium in total, so a single 150mg supplement is a small top-up rather than a big change.
Glycinate, citrate, oxide, taurate and threonate
| Form | How long it stays | Notes |
|---|---|---|
| Glycinate (bisglycinate) | Same as other forms once absorbed | Well tolerated; less laxative effect, so less passes straight through |
| Citrate | Same once absorbed | Well absorbed; more of a laxative effect at higher doses |
| Oxide | Much is unabsorbed and leaves in stools within 1 to 3 days | Lower absorption; often used as a laxative |
| Taurate | Same once absorbed | Little research specific to this form |
| L-threonate | Same once absorbed | Low magnesium content per capsule. See magnesium L-threonate. |
Once magnesium is absorbed, your body handles it the same way whatever form it came from. The form mainly affects how much is absorbed and how your gut tolerates it. See types of magnesium.
Does magnesium build up in your system?
Yes, slowly, if you were low to begin with. Stores in bones and muscles are topped up over weeks of regular intake, not from a single dose. If you’re already well supplied, your kidneys simply pass out the extra. That’s why trials usually run for 4 to 8 weeks, and why taking magnesium daily matters more than the exact time.
How long does magnesium take to work?
- Laxative effect: hours to a day or two. See magnesium for constipation.
- Evening routine and sleep: many people give it two to four weeks. See magnesium for sleep.
- Correcting a low intake: several weeks of daily use.
Full timeline: how long magnesium takes to work. Magnesium contributes to the reduction of tiredness and fatigue and to normal muscle function.
Too much magnesium: how to clear it
For most healthy people, the first sign of too much magnesium from supplements is diarrhoea, and it settles once you stop. The NHS says 400mg or less a day from supplements is unlikely to cause harm.
- Stop or reduce the supplement and any magnesium-containing laxatives or antacids.
- Drink fluids to replace what you lose through diarrhoea.
- Let your kidneys do the work: healthy kidneys clear excess magnesium within about a day.
Dangerously high magnesium levels are rare and usually happen in people with kidney disease, or after very large doses of magnesium laxatives or Epsom salts. Call 111, or 999 in an emergency, if someone has taken a large amount and has drowsiness, weakness, low blood pressure, slow heartbeat, confusion or difficulty breathing. Hospitals can treat it. There’s no safe home “detox” for high magnesium.
Can a blood test tell how much you have?
Only partly. A standard blood test measures the small amount of magnesium in your blood, which your body keeps steady by drawing on stores. So a normal result doesn’t rule out low stores. Your GP will consider your symptoms, diet and medicines too. See signs of magnesium deficiency.
FAQs
How long does magnesium stay in your system?
A single dose is mostly absorbed or passed within a day or two, and excess is cleared by the kidneys within about 24 hours. Body stores build up over weeks.
How long does magnesium glycinate stay in your system?
Much the same as other forms once absorbed: excess is cleared within about a day. Glycinate is gentler on the gut, so less passes straight through.
Does magnesium need to build up in your system?
If you’re low, yes: stores are topped up over weeks of daily use. That’s why consistency matters more than timing.
How do I get rid of excess magnesium?
Stop the supplement, drink fluids, and your kidneys will usually clear it within a day. Get urgent help for weakness, drowsiness or a slow heartbeat.
Can magnesium build up to toxic levels?
Rarely in healthy people. It’s mainly a risk with kidney disease or very large doses of magnesium laxatives.
Should I take magnesium every day?
If you take it, daily is best, within the NHS guide of 400mg or less a day from supplements.
Sources
- Jahnen-Dechent W, Ketteler M. Magnesium basics. Clin Kidney J, 2012.
- de Baaij JH et al. Magnesium in man: implications for health and disease. Physiol Rev, 2015.
- NHS. Vitamins and minerals: magnesium.
