DAVID MCCARTHY / SCIENCE PHOTO LIBRARY DAVID MCCARTHY / SCIENCE PHOTO LIBRARY
Viagra crystals, coloured scanning electron micrograph (SEM). Viagra is the trade name for the drug sildenafil citrate, used to treat male impotence (erectile dysfunction). This drug enhances the smooth muscle relaxant effects of nitric oxide, a chemical that is released in response to sexual stimulation. Relaxation of the smooth muscle allows blood to enter the penis, resulting in erection.
Model release not required. Property release not required.