Erectile dysfunction (ED) affects 20-30 million American men, most of whom are over 50 years of age. In a UK-based study, 32% of British men had difficulty obtaining an erection, 20% with maintaining an erection. In recent years the physiology and pathophysiology of ED have changed our understanding of what ED is from a purely psychological-based disorder to a multifactorial one, with neurological, endocrinological, psychological factors and the role of the vascular system. Recently identified risk factors include diabetes, cardiovascular disease, spinal cord injury, smoking, depression,...