Last updated: October 2026
Yes, some forms of magnesium can make you poo. Magnesium oxide, citrate and hydroxide draw water into the bowel, softening stools, and in a 2021 trial magnesium oxide helped about two thirds of people with chronic constipation. Glycinate has much less effect. Try more fibre, fluids and activity first, and ask a pharmacist before using magnesium as a laxative.
This guide covers how magnesium works on the bowel, which forms to choose, how much, how quickly it works, who should avoid it, and when constipation needs a GP.
How magnesium helps you poo
Some magnesium salts are poorly absorbed, so they stay in the gut and pull water into the bowel by osmosis. That softens stools and makes them easier to pass. This is why magnesium hydroxide and magnesium sulphate (Epsom salts) are sold as laxatives, and why too much magnesium from supplements causes diarrhoea.
What the research shows
In a 2021 Japanese trial, 90 adults with chronic constipation took magnesium oxide (1.5g a day), senna or placebo for four weeks. About 68% improved with magnesium oxide and 69% with senna, compared with 12% on placebo. Magnesium oxide has been used this way for decades, especially in Japan.
Which magnesium is best for constipation?
| Form | Effect on the bowel | Notes |
|---|---|---|
| Oxide | Strong | Best studied for constipation; cheap |
| Hydroxide | Strong | Sold in pharmacies as a laxative |
| Citrate | Moderate | Well absorbed but still loosens stools at higher amounts |
| Sulphate (Epsom salts) | Very strong | Only use as a laxative if a pharmacist or doctor advises |
| Glycinate | Mild | Gentle on the stomach; chosen when you don’t want a laxative effect |
Our Magnesium Glycinate Sleep Complex combines bisglycinate with some citrate and is designed as a gentle evening drink, not a laxative. Compare forms in types of magnesium.
How much and how fast?
- Follow the pack or your pharmacist’s advice for laxative products; they’re for short-term use.
- As a supplement, the NHS says 400mg or less a day of magnesium from supplements is unlikely to cause harm. Higher amounts are more likely to cause diarrhoea.
- Speed: osmotic laxatives often work within one to three days; high doses of magnesium salts can work within hours.
- Drink plenty of water alongside, as magnesium works by drawing water into the bowel.
Side effects and who should avoid it
- Kidney disease: don’t use magnesium laxatives or supplements without your doctor’s advice, as magnesium can build up to dangerous levels.
- Older adults and long-term use: check with a GP; high magnesium levels have been reported with regular magnesium oxide use.
- Medicines: magnesium can reduce absorption of some antibiotics, thyroid and osteoporosis medicines. Leave a gap and ask your pharmacist.
- Side effects: diarrhoea, cramping and wind.
- Pregnancy: only use a laxative recommended by your midwife, GP or pharmacist.
What to try first (NHS)
- More fibre: fruit, vegetables, wholegrains, oats and pulses, increased gradually
- Plenty of fluids, especially water
- Regular exercise, such as a daily walk
- A regular time and place to go, without rushing
- Raising your feet on a low stool while on the toilet
If these don’t help, a pharmacist can suggest a suitable laxative. Constipation is common with weight-loss injections and iron tablets. See magnesium and Mounjaro, magnesium for bloating and CBD for IBS. Wondering about CBD? See can CBD cause constipation?
When to see a GP
See a GP if constipation lasts more than a couple of weeks despite self-help, you need laxatives regularly, or you’re constipated often. Get help urgently if you have blood in your poo, unexplained weight loss, severe tummy pain, a swollen tummy, or a change in bowel habits that lasts several weeks.
FAQs
Does magnesium make you poop?
Some forms do. Magnesium oxide, hydroxide, citrate and sulphate draw water into the bowel and soften stools. Glycinate has much less effect.
What is the best magnesium for constipation?
Magnesium oxide is the best studied; magnesium hydroxide is sold as a laxative. Citrate has a milder effect. Ask a pharmacist which suits you.
How long does magnesium take to work for constipation?
Usually within one to three days. Larger doses of magnesium salts can work within hours.
Is it safe to take magnesium for constipation every day?
Laxatives are meant for short-term use. For regular use, speak to a GP or pharmacist, especially if you’re older or have kidney problems.
Does magnesium glycinate cause diarrhoea?
Rarely at normal amounts. It’s one of the gentlest forms, which is why it’s popular for evening routines.
Can magnesium help bloating and constipation?
Magnesium that loosens stools may ease bloating caused by constipation. Magnesium also contributes to electrolyte balance.
Sources
- Morishita D et al. Senna versus magnesium oxide for the treatment of chronic constipation: a randomized, placebo-controlled trial. Am J Gastroenterol, 2021.
- NHS. Constipation.
- NHS. Vitamins and minerals: magnesium.
- NHS. Laxatives.
