What is the health risk of too much magnesium?

Dietary magnesium does not pose a health risk, however pharmacologic doses of magnesium in supplements can promote adverse effects such as diarrhea and abdominal cramping.

Risk of magnesium toxicity increases with kidney failure, when the kidney loses the ability to remove excess magnesium. Very large doses of magnesium-containing laxatives and antacids also have been associated with magnesium toxicity.

For example, a case of hypermagnesemia after unsupervised intake of aluminum magnesia oral suspension occurred after a 16 year old girl decided to take the antacid every two hours rather than four times per day, as prescribed. Three days later, she became unresponsive and demonstrated loss of deep tendon reflex. Doctors were unable to determine her exact magnesium intake, but the young lady presented with blood levels of magnesium five times higher than normal.

Therefore, it is important for medical professionals to be aware of the use of any magnesium-containing laxatives or antacids.