Can you get too much magnesium?

The upper limit for magnesium applies only to supplements and medicines, not to magnesium in food and drink.

What this means for you

This is the nutrient where the upper limit means something different from what people assume.

Established. The limit applies only to supplements and medicines. Magnesium in food and drink is not capped, because the gut absorbs less of it as intake rises and the kidneys clear the rest. You cannot eat too many pumpkin seeds in the magnesium sense.

Supplements are another story, and the feedback arrives fast. High doses commonly cause diarrhoea with nausea and cramping, which is also why magnesium turns up as the active ingredient in laxatives and antacids. Weigh that against what the dose is buying you. For the commonest reason people take magnesium it buys very little. In a Cochrane review of five trials in 307 older adults with night cramps, the number of cramps a week moved by 0.18 against placebo. If a new supplement has sent you to the bathroom, that is the dose talking, and a smaller one taken with food usually settles it.

Where the 350 comes from

Unsettled. The supplement ceiling has no safety margin inside it. It was set from the lowest dose at which an adverse effect turned up, 360 mg a day, divided by an uncertainty factor of 1.0. Committees normally divide by something larger, to leave room for people more sensitive than the ones who happened to be studied. Here the number you are told to stay under sits ten milligrams below the number where trouble was recorded.

Unsettled. The trouble in question is diarrhoea, and most of the studies behind the ceiling never said what they were counting as diarrhoea. Where it was written down at all it was usually self-reported and loosely described, against a standard definition of loose watery stools three or more times a day. A limit can rest on a soft endpoint and still be a sensible limit. What it means here is that 350 is a rounder number than it looks, and that crossing it is a bathroom event rather than a poisoning.

Two habits make a high dose easier to live with, and neither of them is on the label. Split it across the day instead of swallowing it in one go, and take it with food. The salt matters as well. Magnesium gluconate and magnesium chloride cause diarrhoea less often than the other forms, carbonate is too insoluble to absorb properly, and citrate dissolves readily enough to move the bowels more than oxide does.

What the side effects actually measure

Across the trials of magnesium for cramps, minor side effects were about half as common again on magnesium as on placebo, a risk ratio of 1.51, and almost all of them were in the gut. Serious events were no more common: two in 72 people on magnesium against three in 68 on placebo. In a six-month trial of 450 mg a day in overweight adults with working kidneys, the one serious event reported was judged to have nothing to do with the treatment, and blood magnesium moved by 0.04 millimoles per litre over that half year. That is close to nothing.

The dose that does reach your gut is used on purpose. In people with long-standing constipation, 1.5 g a day of magnesium oxide brought overall improvement in 68 per cent of them against 12 per cent on placebo, which is about what senna managed, with no severe side effects over four weeks. Same mineral, laxative dose. That is where the diarrhoea in the paragraph above comes from.

Not for everyone. The picture changes completely if your kidneys are impaired, and there is now a trial to point at rather than a mechanism. People with chronic kidney disease took magnesium hydroxide for a year. Thirty-five of them had gut trouble against nine on placebo. There were five deaths and six cardiovascular events in the magnesium group against two deaths and none on placebo, and the artery calcification the trial set out to slow did not move. Anyone with reduced kidney function should treat magnesium supplements as a medical decision rather than a shelf decision, including the magnesium hiding in antacids and laxatives.

Unsettled. More is not better even inside the normal range, and the data that suggests it comes from sick people. Among 6,147 outpatients with heart failure, those whose blood magnesium sat above 0.95 millimoles per litre had the worst event rate of the three groups, worse than those below 0.75. This is observational data collected inside a trial, so it cannot tell you whether the high magnesium did any harm or simply marked something else that did. What it does rule out is reading a high result as a good one.

Worth weighing the dose against what it is for. The whole randomised evidence for magnesium and sleep is three trials in 151 older adults, who fell asleep about 17 minutes sooner, on evidence rated low to very low. That is not a result worth pushing a dose upward for, and pushing it upward is exactly what produces the diarrhoea described above.

Two interactions are about timing rather than about dose. Magnesium binds tetracycline and quinolone antibiotics into insoluble complexes, and it reduces absorption of oral bisphosphonates taken for osteoporosis. A few hours between the tablets solves both, and the magnesium hiding in antacids and laxatives counts for this purpose too.

Worth knowing what the dose is for before pushing it up. The strongest recent result is in depression: seven randomised trials in 325 adults with a depressive disorder found lower depression scores on magnesium, and the authors immediately add that the trials are too small to act on clinically. That is not a reason to take more than the gut will tolerate.

Written from the evidence cards and the data on this page, then checked back against the sources. Bioma Learn has no human medical reviewer at this time and we say so rather than invent one. Information, not medical advice. How we verify.

Upper limits apply to everything you take in, from supplements and, for most nutrients, from food as well. Reaching one from whole food alone is uncommon. Supplements are where people cross the line, usually without meaning to.

Full guide: Magnesium: widely under-eaten, and widely over-promised