A 39-year-old women was admitted to the hospital following a large ingestion of a tricyclic antidepressant. The administration of magnesium citrate in repeated doses with activated charcoal resulted in a striking increase in serum magnesium levels followed by acute neuromuscular deterioration and respiratory depression. The patient required dialysis for control of hypermagnesemia. Her clinical condition improved slowly without further complication and she was discharged to a rehabilitation center.