Magnesium: What It Does, Which Form to Take, and How Much
Close to half of Americans eat less magnesium than recommended, and the benefits concentrate in people who were short to begin with. Beyond that, magnesium is a dependable laxative and a modest help for migraine and blood pressure. It does little for sleep, and the best trials find nothing for the night leg cramps it is most often sold for.
Magnesium is a mineral used throughout the body's cells, most of it stored inside cells and bone, with very little in the blood. You get it from leafy greens, nuts, seeds, legumes, and whole grains, and a diet light on those runs low. A few common medicines, including proton pump inhibitors and diuretics, lower it further. Because so little magnesium circulates in blood, a standard blood test can read normal even when whole-body stores are low.
The evidence sorts cleanly by how strong it is. As an osmotic laxative, magnesium works well and fast: in trials of magnesium oxide for constipation, about 70% improved against 25% on placebo. Daily high-dose magnesium is rated possibly effective for migraine prevention. Across 34 randomized trials it lowered systolic blood pressure by about 2 mmHg, more in people who started low. People who eat more magnesium also have less heart failure and diabetes over the years. That comes from cohort data, which shows a pattern but cannot prove the mineral alone drives it.
For sleep, the pooled evidence is three small trials in older adults, where magnesium shaved about 17 minutes off the time it took to fall asleep, on evidence the reviewers rated low to very low quality. For anxiety, roughly half of trials in anxiety-prone people showed a benefit and half showed nothing, and most of those trials paired magnesium with vitamin B6, so magnesium alone cannot take the credit. For the night leg cramps it is marketed for hardest, a Cochrane review found no meaningful benefit over placebo in older adults.
Food comes first: a magnesium-rich diet brings fiber, potassium, and plant compounds along with the mineral. If you supplement, match the form to the goal: magnesium glycinate or citrate for raising body magnesium, magnesium oxide only if you specifically want the laxative effect, since it is poorly absorbed. A typical dose is 200 to 400 mg of elemental magnesium a day with food; loose stools mean you have gone past what you need. Anyone with reduced kidney function should check with a clinician first, since the kidney is what clears the excess.