Can you take too much magnesium?
Yes, too much magnesium can be dangerous, although serious toxicity from ordinary supplements is rare in people with healthy kidneys. With kidney problems or when using intravenous magnesium, medical supervision is genuinely necessary.
Oral magnesium taken as a supplement or laxative can indeed raise your serum magnesium. In almost half of the patients who repeatedly received magnesium citrate as a laxative, blood magnesium rose above normal values. Seriously dangerous levels were not reached in those cases, but this took place in a medical setting with close monitoring. Without such oversight, the likelihood of problems is harder to estimate.
The real dangers lie with intravenous magnesium, which goes directly into the bloodstream. One case report describes serious neurological deterioration caused by an overdose administered via an infusion, and in pregnancy-related hypertension (eclampsia) an overdose of intravenous magnesium is such a genuine risk that doctors check the knee-jerk reflex every hour: it disappears as an early warning sign before the situation becomes truly dangerous. These kinds of risks do not apply to ordinary magnesium supplements bought at a pharmacy or health store.
Yet too little magnesium is also dangerous. A magnesium deficiency can cause or worsen cardiac arrhythmias, particularly in people who use certain heart medications or diuretics. The reason: without sufficient magnesium, the cell also loses potassium, and that cannot be corrected simply by replenishing potassium alone.
In short: with ordinary supplements at normal doses, serious toxicity is unlikely for healthy people, but at high doses or in conditions where the kidneys excrete magnesium less efficiently (such as kidney problems) levels can build up. Intravenous magnesium is a different matter entirely: it requires medical supervision.
Based on five claims: one observational study in laxative users, one case report on intravenous overdose, one clinical guideline on eclampsia treatment, one limited association study on cardiac arrhythmias, and one clinical-experimental claim on magnesium deficiency. No RCTs specifically examining oral supplement overdose in healthy individuals.