Which of the following is NOT an expected side effect associated with Clonidine (Catapres)?

• A. Increase IOP
• B. Decrease vascular resistance
• C. Decrease heart rate
• D. Increase parasympathetic tone

Answer: (A)

Clonidine works by activating alpha-2 receptors in the brain, which reduces sympathetic outflow. That central calming of the sympathetic system lowers peripheral vascular resistance and blood pressure, and it tends to slow the heart rate. It also shifts the autonomic balance toward parasympathetic activity, contributing to bradycardia and reduced vascular tone.

An expected effect of this mechanism is a decrease in vascular resistance, and a slower heart rate, with a relative increase in parasympathetic influence. These align with how clonidine reduces BP and heart rate clinically. Intraocular pressure, however, is not increased by clonidine; alpha-2 agonists can actually lower aqueous humor production, and topical versions are used to reduce IOP in glaucoma. Therefore an increase in intraocular pressure is not a typical side effect you’d expect from clonidine.

So the option describing an increase in intraocular pressure would not fit clonidine’s profile, while the other listed effects are consistent with its central sympatholytic action.