Andrex is an androgen that is used to treat impotence and other male sexual issues such delayed puberty that result from low testosterone in men as well as hypogonadism, or testosterone insufficiency. A disorder known as hypogonadism occurs when the testes do not generate enough sperm, testosterone, or both. Andrex contains testosterone undecanoate which works by increasing testosterone production in the body which can help increase sperm count and boost libido (sex drive).