During magnesium sulfate therapy, which finding indicates possible toxicity?

• A. Hyperreflexia
• B. Tetany
• C. Respiratory depression
• D. Decreased deep tendon reflexes

Answer: (D)

Magnesium sulfate acts as a CNS depressant and muscle relaxant to prevent seizures, but as levels rise, it begins to blunt neuromuscular transmission. The earliest sign of magnesium toxicity is a loss or reduction of deep tendon reflexes. When reflexes become diminished or absent, it signals that magnesium is already at a level high enough to impair neuromuscular function, so the infusion should be reassessed and stopped if needed, with close monitoring of airway and breathing.

As toxicity progresses, respiratory depression can develop due to weakened respiratory muscles, which is a more serious and later consequence. Hyperreflexia would not indicate toxicity; it points toward CNS irritability, often seen with preeclampsia. Tetany suggests low magnesium or calcium, not toxicity. So the decrease in deep tendon reflexes is the best, most reliable early warning sign of magnesium toxicity.