Can you take too much magnesium?
Yes, too much magnesium can be harmful, particularly through intravenous administration or repeated use of laxatives. With common oral supplements at normal doses the risk appears small, but people with kidney problems would do well to discuss this with their doctor.
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Magnesium from food rarely causes problems, because healthy kidneys excrete any excess quickly. With magnesium supplements or medicines it is a different story: the amount can rise to a level your body can no longer process easily.
Magnesium citrate used as a laxative is a good example. In 102 patients who took it repeatedly, blood magnesium levels rose noticeably: in nearly half of them levels climbed above the normal upper limit, and in 12 of the 102 patients they reached clearly elevated values. Dangerously high levels fortunately did not occur, but this shows that you cannot simply ignore blood levels when using these kinds of products on a regular basis.
Intravenous magnesium, such as doctors administer for a serious pregnancy complication, requires precise dosing. A well-known safety check in this context is the knee-jerk reflex: it is the first thing to disappear once magnesium levels become too high, before truly dangerous effects set in. Doctors use this as an early warning signal.
What can go wrong with an excessive dose is illustrated by a documented medication error: a 65-year-old man accidentally received too much magnesium intravenously and developed acute neurological symptoms. Blood tests confirmed that his magnesium level was strongly elevated. This is a rare situation outside a hospital setting, but it makes clear that magnesium at high doses is certainly not harmless.
No serious problems emerged from these studies for daily use of oral supplements at common doses. Nevertheless, it is wise not to use magnesium-containing laxatives for prolonged periods without medical advice, especially if you have kidney problems, because the kidneys are responsible for clearing the excess.
Three studies, all focusing on medical use (laxative, eclampsia treatment, medication error); no RCTs on oral supplementation in healthy people. Evidence is limited but causal for the effects found.