Which statement about theophylline is true?

• A. It has a wide therapeutic window and minimal side effects
• B. It is a diuretic with renal clearance
• C. It is safe in all ages
• D. It has a narrow therapeutic window and risk of severe adverse reactions

Answer: (D)

The main concept being tested is theophylline’s therapeutic index and toxicity risk. Theophylline is a methylxanthine bronchodilator with a narrow therapeutic window, meaning the dose needed to achieve benefit is close to the dose that causes harmful effects. In practice, therapeutic serum levels are modestly spaced from toxic levels, but patient factors can shift clearance dramatically. Adults commonly have a therapeutic range around 10–20 mcg/mL, yet toxicity can appear with levels not far above that, presenting with symptoms such as tachycardia, arrhythmias, tremor, nausea, and in severe cases seizures. Liver metabolism, particularly via CYP1A2, creates substantial interindividual variability; drug interactions (like macrolide antibiotics, cimetidine, or fluoroquinolones) can raise levels, while factors such as smoking increase clearance and reduce effect. Age-related changes and liver disease can also tilt the balance toward toxicity, which is why careful monitoring and dose adjustments are essential.

Because of these factors, saying it has a wide therapeutic window is incorrect, and it isn’t safe in all ages without monitoring. While it may have some diuretic effects, that is not its defining or primary clinical feature, and the key point is the narrow window and toxicity risk.