Magnesium for constipation: how it works and how to take it

How does magnesium relieve constipation?

Magnesium salts are osmotic laxatives. The bowel absorbs only part of a large dose of magnesium, and the rest stays in the gut, where it holds water. That softens the stool and makes it easier to pass. This is the same reason high-dose magnesium supplements often cause loose stools (see side effects). The least-absorbed forms, such as magnesium oxide, have the strongest effect; citrate is milder.

What is the evidence for magnesium as a laxative?

Unlike most magnesium claims, this one rests on long clinical use as well as trials. Magnesium hydroxide and magnesium sulphate are sold as licensed laxatives in UK pharmacies. Epsom salts are magnesium sulphate, but bath products aren’t meant to be swallowed: see Epsom salts and sprays.

The best recent trial, from Japan in 2021, randomly gave 90 adults with long-standing constipation either magnesium oxide (1.5 g a day), senna or placebo for four weeks. Overall symptoms improved in 68% on magnesium oxide, 69% on senna and 12% on placebo. Bowel movements and quality of life also improved, and there were no serious side effects. A 2021 review of magnesium oxide for constipation reached a similar conclusion, while stressing the need to monitor blood magnesium in long-term users.

How do you take magnesium for constipation?

• Follow the directions on a licensed laxative, or ask a pharmacist to recommend one.
• Drink plenty of fluids.
• Use the lowest dose that works, and don’t keep using it for more than about a week without asking a pharmacist or GP.
• More fibre, fluids and activity are the NHS’s first suggestions for constipation, and are worth trying alongside.

When should you avoid magnesium laxatives?

Don’t use magnesium laxatives if you have kidney disease, unless your doctor advises it. Take extra care if you are older, because kidney function falls with age. Very high blood magnesium (hypermagnesaemia) is rare but can be fatal: a 2018 case report described a woman with normal kidney function who died after long-term magnesium laxative use, and the authors noted that constipation itself may let more magnesium be absorbed.

Magnesium can also stop some medicines being absorbed, so check the medicines it affects.

See your GP if constipation is new and lasts more than three weeks, if you see blood, have unexplained weight loss or tummy pain, or if laxatives don’t help.

What doesn’t the evidence show?

That gentle forms sold for sleep or anxiety, such as glycinate, relieve constipation reliably. They are designed to avoid exactly this effect.