Testosterone Undecanoate 40mg is used for testosterone replacement therapy in adult males with low or absent natural testosterone due to conditions such as primary or secondary hypogonadism. This medication helps restore normal testosterone levels, which supports the development and maintenance of male secondary sexual characteristics, muscle mass, bone density, and sexual function. It is also used for symptoms related to low testosterone like fatigue and sexual dysfunction. The medication is available as oral capsules and is absorbed via the lymphatic system, making it effective for hormone replacement.