A patient taking sildenafil should not take which medication because of a contraindication?

• A. Isosorbide
• B. Phenytoin
• C. Metronidazole
• D. Prednisone

Answer: (A)

Sildenafil is a PDE5 inhibitor that increases levels of cGMP to promote vasodilation. When it’s taken with nitrates, such as isosorbide, the nitrate donates nitric oxide, which greatly boosts cGMP. Sildenafil then prevents the breakdown of that cGMP, leading to an excessive drop in blood pressure. This combination can cause dangerous hypotension, fainting, and even heart complications, so it’s a contraindication to take them together.

The other drugs listed don’t create that same nitrate-induced blood pressure drop with sildenafil, so they aren’t contraindicated for this reason. Phenytoin might affect sildenafil levels via enzyme induction, and prednisone or metronidazole don’t have the same acute hemodynamic interaction with PDE5 inhibitors.