The kidneys are efficient at excreting excess magnesium and it is unlikely that the mineral will accumulate to toxic levels. A high intake of magnesium might impair absorption and use of calcium.

Though difficult to achieve through dietary means alone, symptoms of magnesium overload include diarrhea, difficulty breathing and depression of the central nervous system, causing muscle weakness, lethargy, sleepiness, or even hyperexcitability. In extreme states, magnesium overload can cause death. Old age is often associatted with reduced kidney function. This increases the likelihood of magnesium overload in the aged.