Erections are achieved via a complex pathway beginning with stimulation of nerves which increase blood flow into the penis. Cylinders inside the penis called corpora cavernosa expand, compressing veins that drain blood away from the penis. This causes the penis to enlarge and become rigid. Impotence can occur if the nerves do not function properly, if not enough blood enters the penis, or if the veins allow too much blood to leak out of the penis. Neurologic problems that can cause impotence include strokes, Parkinson's disease, multiple sclerosis or spinal cord injuries.
Some operations can also injure the nerves or blood vessels responsible for erections.
Atherosclerosis can also cause diminished blood flow to the penis, resulting in the inability to achieve erections. Risk factors for atherosclerosis include smoking, hypertension, high cholesterol and diabetes. Another cause of vascular impotence is trauma or injury to key blood vessels. Many medications can also cause impotence, including certain high blood pressure medications, antipsychotic drugs, antidepressants, and sedatives, including alcohol and marijuana. Hormonal problems from disorders of the testicles, thyroid, adrenal or pituitary gland occasionally cause decreased erections.