Many abusers who inject anabolic steroids may use nonsterile injection techniques or share contaminated needles with other abusers. In addition, some steroid preparations are manufactured illegally under nonsterile conditions. These factors put abusers at risk for acquiring lifethreatening viral infections, such as HIV and hepatitis B and C. Abusers also can develop endocarditis, a bacterial infection that causes a potentially fatal inflammation of the inner lining of the heart. Bacterial infections also can cause pain and abscess formation at injection sites.
In rodents, estrogens (which are locally aromatized from androgens in the brain) play an important role in psychosexual differentiation, for example, by masculinizing territorial behavior;  the same is not true in humans.  In humans, the masculinizing effects of prenatal androgens on behavior (and other tissues, with the possible exception of effects on bone) appear to act exclusively through the androgen receptor.  Consequently, the utility of rodent models for studying human psychosexual differentiation has been questioned. 
Anabolic and androgenic steroids are available as prescription medications to be used in cases in which the body does not make enough hormone and supplementation may be required. Some hormone supplements in this pathway include growth hormone and testosterone itself. These medications are legally prescribed by health-care providers, but this group of drugs is often used illegally and abused to help increase athletic performance and improve body appearance. When used in a well-nourished body, anabolic steroids will cause weight gain primarily due to an increase in muscle mass.