Phosphodiesterase inhibitors are a class of medications that block the action of phosphodiesterase enzymes. These enzymes break down cyclic nucleotides, such as cyclic AMP and cyclic GMP, which are important for various cellular functions. By inhibiting these enzymes, these drugs increase the levels of cyclic nucleotides, leading to enhanced signaling pathways that can improve blood flow, reduce inflammation, and promote relaxation of smooth muscles. Commonly used in treating conditions like erectile dysfunction, pulmonary hypertension, and certain heart diseases, phosphodiesterase inhibitors can help improve symptoms and overall quality of life. Examples include sildenafil, tadalafil, and vardenafil, each with specific uses and effects.