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Behind the scenes, a lot goes into achieving an erection. When you're turned on, nerves fire in your brain and travel down your spinal cord to your penis. There, muscles relax and blood flows into the blood vessels. The result, if all goes well, is a rigid penis, ready for sex.

Unfortunately, all does not always go well. Many diseases -- and, in some cases, their treatment -- can lead to erectile dysfunction (ED). So can injuries, lifestyle choices, and other physical factors. ED can often be treated, and finding the right cause can lead to successful treatment.

The physical causes of ED are not only disease-related. There are many other potential causes, including:

  • Surgery: Surgery for both prostate cancer and bladder cancer can damage nerves and tissues necessary for an erection. Sometimes the problem clears up, usually within 6 to 18 months. But the damage can also be permanent. If that happens, treatments exist to help restore your ability to have an erection.
  • Injury: Injuries to the pelvis, bladder, spinal cord, and penis that require surgery also can cause ED.
  • Hormone problems: Testosterone and other hormones fuel a man's sex drive, and an imbalance can throw off his interest in sex. Causes include pituitary gland tumors, kidney and liver disease, depression, and hormone treatment of prostate cancer.
  • Venous leak: To keep an erection, the blood that flows into your penis must stay in your penis. If it flows back out too quickly -- a condition called venous leak, in which the veins in your penis don't constrict properly -- you will lose your erection. Both injuries and disease can cause venous leak.
  • Tobacco, alcohol, or drug use: All three can damage your blood vessels. That makes it difficult for blood to reach the penis, which is essential for an erection. If you have hardened arteries (arteriosclerosis), smoking will dramatically raise your risk of ED.
  • Prescription drugs: There are more than 200 prescription drugs that can cause ED.
  • Prostate enlargement: Prostate enlargement, a normal part of aging for many men, may also play a role in ED.