Testosterone belongs to a class of drugs known as androgens. Testosterone cypionate is used for males who do not produce enough of a natural hormone known as testosterone. In males, testosterone is responsible for numerous physiological activities, including genital, muscle, and bone growth and development. It also helps produce proper sexual development puberty in males. This medication is given by injection into the buttock muscle as directed by your doctor, usually every 1 to 4 weeks. The more common side effects of testosterone cypionate can include pain and swelling at the injection site, hair growth, headache, and mood swings.