For example, low levels of testosterone or thyroid hormone often cause poor quality erections. Excessive production of prolactin by the pituitary gland may contribute to a low testosterone level and lack of desire. Diabetes is also considered an endocrine disease. Prescription drugs often cause Impotence as a side effect, and over 200 medications fall in to this category. Never change a dosage or stop taking a prescribed drug without the advice of your doctor.
Substance abuse affects erectile function as well.
Illegal drugs and the excessive use of alcohol or cigarettes can seriously damage the blood vessels and nerves involved in a normal erection.