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 quickly pass out any excess in urine, 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 | Somewhere between about a quarter and three-quarters of the magnesium you take in is absorbed, mostly in the small intestine. The lower your levels, the more you absorb. | Over several hours |
| In the blood | Only about 1% of your body’s magnesium is outside your cells, and just 0.3% is in your blood serum. Blood levels are kept within a tight range. | Constantly balanced |
| Into tissues | About 99% of your magnesium is in bones, muscles and soft tissues, with roughly 50 to 60% in bone. | Builds up over weeks |
| Kidneys | The kidneys filter about 2,400mg of magnesium a day and immediately reabsorb about 95%, so only around 100mg leaves in urine. They can raise or lower this quickly depending on how much you take in. | Excess passed out quickly, mostly at night |
| Unabsorbed magnesium | Leaves in your stools, and can loosen them. | With your next bowel movements |
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.
Magnesium in your body: a timeline
| Time after you start | What’s going on |
|---|---|
| First few hours | The dose is absorbed through the small intestine |
| Same day and night | Blood levels stay steady; your kidneys pass out what you don’t need, with most excretion at night |
| Next bowel movements | Unabsorbed magnesium leaves in your stools, which is why high doses can loosen them |
| 2 to 8 weeks of daily use | If you were low, stores in bone and muscle are gradually topped up. Most trials ran for this long |
| After you stop | Your body keeps drawing on its stores and adjusts how much it absorbs and excretes, so levels don’t crash overnight |
What makes you lose magnesium faster?
Some things increase how much magnesium you lose, or reduce how much you absorb. A medical review lists:
- Long-lasting or severe diarrhoea.
- Some medicines, including loop diuretics (water tablets such as furosemide) and proton pump inhibitors such as omeprazole. The MHRA has warned that long-term PPI use, especially for over a year, can cause low magnesium.
- Heavy drinking over a long time.
- Diabetes.
- Prolonged exercise, heat and heavy sweating.
If any of these apply and you have symptoms such as cramps, twitching or tiredness, speak to your GP or pharmacist. Don’t stop prescribed medicine on your own. See signs of magnesium deficiency and electrolytes.
Magnesium sprays and Epsom salt baths: how much gets in?
Probably very little. A 2017 review of the evidence concluded that the idea of magnesium being absorbed through the skin from sprays, flakes or bath salts is scientifically unsupported. A warm bath or massage can still be a relaxing part of an evening routine, but if you want to top up your magnesium, food and oral supplements are the proven routes. See magnesium spray.
Do you need to take magnesium every day?
Because excess is passed out quickly and stores change slowly, steady daily intake works better than occasional large doses. Spreading larger amounts across the day can also be gentler on your gut. The NHS says adults need 300mg (men) or 270mg (women) a day, most of which can come from food such as spinach, nuts and wholemeal bread, and 400mg or less a day from supplements is unlikely to cause harm. See foods high in magnesium and the best time to take magnesium.
Our Magnesium Glycinate Sleep Complex is a daily berry evening drink with magnesium bisglycinate and citrate plus L-theanine. 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 can quickly increase how much magnesium they pass out.
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.
Magnesium and your kidneys
Your kidneys are what stop magnesium building up, so kidney health matters most for safety. If you have kidney disease, don’t take magnesium supplements, magnesium laxatives or antacids unless your doctor advises it, because your body can’t clear the excess properly. The same applies to Epsom salts taken by mouth or as an enema, which have caused dangerous magnesium levels.
FAQs
How long does magnesium stay in your system?
A single dose is mostly absorbed or passed within a day or two, and your kidneys quickly pass out any excess. Body stores build up over weeks.
How long does magnesium glycinate stay in your system?
Much the same as other forms once absorbed: your kidneys pass out the excess. 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 and drink fluids; healthy kidneys will clear it. 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.
How long does magnesium citrate stay in your system?
Once absorbed, the same as other forms. Citrate has more of a laxative effect, so at higher doses some passes straight through with your next bowel movements.
Do Epsom salt baths raise your magnesium?
Probably not much. A 2017 review found the evidence for magnesium being absorbed through the skin is unsupported. Food and oral supplements are the proven routes.
Does alcohol make you lose magnesium?
Heavy drinking over a long time is a known cause of low magnesium. Cutting down helps your body hold on to it.
What medicines lower magnesium?
Long-term proton pump inhibitors such as omeprazole and some diuretics (water tablets) can lower magnesium. Speak to your GP or pharmacist rather than stopping them.
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.
- Gröber U et al (2017). Myth or reality: transdermal magnesium? Nutrients.
- MHRA (2012). Proton pump inhibitors in long-term use: reports of hypomagnesaemia. Drug Safety Update.
This guide is general information, not medical advice. Evopure makes the Magnesium Glycinate Sleep Complex.
