Polarized light micrograph of sildenafil citrate (trade names: Viagra, Revatio, etc.) crystals, a phosphodiesterase 5 inhibitor. As a vasoactive drug, it is used to treat erectile dysfunction. It elevates levels of cGMP, which regulates blood flow to the penis, by inhibiting its breakdown via phosphodiesterase type 5, found in particularly high concentrations in the corpus cavernosum (erectile tissue of the penis). It is widely recognized in popular culture due to the belief that it improves sexual performance and increases libido and penis size. As an antihypertensive agent it is used to treat pulmonary arterial hypertension. It improves exercise capacity in both men and women. Sildenafil has potential for the treatment of Alzheimer's disease and other diseases associated with elevated amyloid beta-peptide levels. Field of view size: 1.485 x 2.227 mm (0.0585 x 0.0877 in)