Exactly how low serum testosterone contributes to erectile dysfunction is still unclear. With the patches the peak level of testosterone occurs between two and six hours after the patch application, followed by a gradual decrease in the next 24 hours; therefore, the patches have to be applied daily. The most common adverse events described have been itching (7 percent), chronic skin irritation (2 to 5 percent), and allergic dermatitis (4 percent). With a transdermal patch normal serum testosterone levels can be achieved in more than 90 percent of patients. They are more reliable in maintaining a more natural testosterone level and more closely mimic the normal physiologic testosterone secretion. An office visit for an injection is not required.

The transdermal preparations, Testoderm and Androderm, are more convenient. On the other hand, in patients with low serum testosterone levels who also have erectile dysfunction, the evaluation of the effects of an injection of Depo-Testosterone takes only a few days. It is therefore far from an ideal physiologic replacement. It has the obvious disadvantage of peaks and valleys in blood levels of testosterone. The esterified testosterone products, Depo-Testosterone, and testosterone enanthate, are given with intramuscular injections at doses of 200 to 400 mg every three to four weeks. For patients who cannot take the injectable form of testosterone, or cannot use the patches, the oral methyl testosterone at doses of 25 to 50 mg per day may be used as long as there is close monitoring of liver function. From a clinical standpoint, however, the effectiveness of hormone replacement in men with low testosterone levels has been rather disappointing. However, a recent reportsuggests there may be a direct effect of testosterone on the erectile tissue. It may be more of a central mechanism affecting libido than affecting penile tissue itself.