Esters such as testosterone cypionate and testosterone enanthate can be injected every one to three weeks. Since the esters are hydrolysed before the hormones act, the effectiveness of therapy can be monitored by measuring the plasma testosterone level following administration. At physiologic replacement doses, testosterone esters have no known side effects in mature men. At supraphysiologic doses, however, gonadotrophin secretion is inhibited, the testes decrease in volume, and the sperm count falls (indeed, low sperm counts may persist for as long as nine months after supraphysiologic doses are discontinued). The administration of testosterone esters results in an increase in plasma oestrogen levels and may, on occasion, cause gynaecomastia.
