Dietary magnesium very rarely poses a health risk, but very high doses of magnesium supplements, which may be added to laxatives for example, can promote adverse effects such as diarrhea. The kidneys are efficient at excreting excess magnesium and it is unlikely that the mineral will accumulate to toxic levels.
The Institute of Medicine of the National Academy of Sciences has established a tolerable upper intake level (UL) for supplementary magnesium
for adolescents and adults at 350mg daily.
As intake increases above the UL, the risk of adverse effects increases. In extreme cases, this can mean coma or even death.