Hypermagnesemia (taking too much magnesium) is rare with supplements in individuals with normal kidney function, as the kidneys can filter out excess magnesium. However, overdose is possible, especially with severe kidney impairment. Symptoms can include mild gastrointestinal discomfort (diarrhea, nausea, abdominal cramps), headache, dizziness, drowsiness, thirst, low blood pressure (hypotension), confusion, muscle weakness, loss of tendon reflexes, urinary retention, depression, irregular heartbeat, respiratory depression, coma, and in severe cases, cardiac arrest and death.
In case of accidental overdose with serious symptoms (passing out, trouble breathing, changes in mental status, irregular heartbeat), immediately seek emergency medical attention. For less severe symptoms, contact a poison control centre or your healthcare provider.