Testosterone undecanoate (undestor, andriol, nebido) are used to replace the body's natural sex hormone testosterone when not enough is made by the body. Testosterone undecanoate is a long-acting man-made version of testosterone, the natural male sexual hormone. Testosterone is responsible for the normal growth and development of male sex organs and characteristics. It includes growth and development of male organs of penis, testicles, prostate, body hair, vocal cord thickening, and muscle and fat distribution.