Theophylline with cimetidine increases the risk of what?

• A. No interaction
• B. Theophylline toxicity risk increased
• C. Decreased theophylline effect
• D. Theophylline toxicity risk increased only with high dose

Answer: (B)

The key idea is that theophylline is cleared by liver enzymes, notably CYP1A2, and has a narrow therapeutic window. Cimetidine inhibits these hepatic enzymes, so theophylline levels rise in the blood. Higher theophylline levels increase the risk of toxicity, with symptoms like nausea, vomiting, tremor, tachycardia, and in severe cases seizures or arrhythmias. Therefore, the interaction is best described as an increased risk of theophylline toxicity, not a change in efficacy, not no interaction, and it isn’t limited to high doses— toxicity risk can occur even at standard doses when cimetidine is also used.