SUSTANON Testosterone is a male sex hormone that is responsible for the development of male characteristics. These include growth of hair on the face and body, deepening of the voice, muscle development and the development of the male genitalia. Testosterone is also important for maintaining the strength of bones, libido and fertility in men. Men who do not produce enough testosterone may be prone to problems such as decreased libido, infertility or osteoporosis. In men who do not produce enough testosterone, Sustanon 250 is used to reduce the effects of a low level of testosterone.