What is the earliest sign of magnesium toxicity?

• A. Hyperreflexia
• B. Tachycardia
• C. Absent or decreased deep tendon reflexes
• D. Hypertension

Answer: (C)

Magnesium toxicity first affects neuromuscular function. When magnesium levels rise, the neuromuscular system becomes progressively more suppressed, and the earliest clinical sign is a loss or decrease of deep tendon reflexes. This happens before other symptoms like changes in heart rate or blood pressure, which tend to occur later.

So, the absence or reduction of deep tendon reflexes is the best warning sign to watch for in patients receiving magnesium sulfate. If this is observed, stop the magnesium infusion, assess respiratory status, and notify the clinician. Be prepared to use the available antidote, calcium gluconate, if ordered, and monitor for progression to hypotension, bradycardia, or respiratory depression.