The National Institutes of Health states that diarrhea, nausea and abdominal cramps are among the effects of consuming too much magnesium. Magnesium toxicity can occur in very large doses of over 5,000 mg per day and may lead, in extreme cases, to cardiac arrest.
The source notes that negative effects of consuming magnesium are only likely to occur when individuals ingest the mineral via dietary supplements or medication. Healthy persons are not at risk of consuming too much magnesium from food due to the kidneys' elimination of excess amounts of the mineral in urine. Persons with impaired renal function or kidney failure are at higher risk of magnesium toxicity.