Which of the following is a sign of magnesium sulfate toxicity?

• A. Respiratory depression with respiratory rate <14
• B. Elevated deep tendon reflexes
• C. Hypertension and tachycardia only
• D. Hyperglycemia with improved mental status

Answer: (A)

Magnesium sulfate, used for seizure prophylaxis, can accumulate and depress the nervous system and respiration. A hallmark sign of toxicity is respiratory depression, shown by a slower-than-normal breathing rate (for example, fewer than about 14 breaths per minute). This rapid change can progress to respiratory failure, so recognizing it promptly and stopping the infusion and notifying the provider is critical.

As magnesium rises, reflexes typically diminish rather than increase—loss or markedly decreased deep tendon reflexes are an important early warning. Hemodynamic changes usually lean toward hypotension and bradycardia rather than hypertension and tachycardia. Hyperglycemia with improved mental status doesn’t fit the pattern of magnesium toxicity either. In practice, monitoring includes respiratory status, reflexes, urine output, blood pressure, and serum magnesium level, with calcium gluconate available as the antidote if toxicity occurs.