Low Testosterone Levels
Testosterone is necessary for sex drive (libido), sperm production, and maintaining nitric oxide levels in the penis. Therefore men with hypogonadism (poor function of the testes which results in lowered testosterone production) may display low sex drive, poor sperm production, and erectile dysfunction. This is a routine test that can be ordered by your doctor.

Mental-Emotional Conditions
Psychological factors may be responsible for erectile dysfunction. Some common afflictions include anxiety, depression, guilt, low self-esteem, and performance fears. Negative feelings that you express toward your sexual partner ?or that are expressed by your sexual partner ?such as resentment, hostility or lack of interest also can be a factor in erectile dysfunction.

Habitual Substance Use
Alcohol can cause nerve damage and atrophy (shrink) the testicles leading to lowered testosterone production. Marijuana, heroin, cocaine, ecstasy, methamphetamines, and other recreational substances can also contribute to erectile dysfunction.

Medications
Erectile dysfunction can be caused by many common medications. Some of these include drugs for hypertension, allergies and congestion, depression and anxiety, as well as some tranquilizers, and appetite suppressants.