The Nature of Testosterone Replacement Therapy: Testosterone is a key hormone in both men and women, though men have higher levels of it than women do. All men experience a decline in testosterone in middle age, and in some that produces fatigue or loss of sexual desire. As a result some men seek testosterone-replacement therapy, in which a steady dose of additional testosterone is supplied. Pills, patches, topical gels, or injections are used to deliver the hormone, and millions of men have been treated with AndroGel, which is sold under the names Striant, Androderm, and Testim.
