mixtures containing lead,arsenic,or strychnine.Substances currently favored by practitioners of herbal medicine include ginseng and kava. Scientific studies of the process have shown that erection requires relaxation of the nonvascular smooth muscle of the corpora cavernosa.This relaxation permits inflow of blood at nearly arterial pressure into the sinuses of the cavernosa,and it is the pressure of the blood that causes erection.Physiologic erection occurs in response to the release of nitric oxide from nonadrenergic-noncholinergic nerves associated with parasympathetic discharge.Thus,parasympathetic innervation must be intact and nitric oxide synthesis must be active.(It appears that a similar process occurs in female erectile tissues.)Certain other smooth muscle relaxants eg,PGE analogs or antagonists if present in high enough concentration,can independently cause sufficient cavernosal relaxation to result in erection.As noted in the text,nitric oxide activates guanylyl cyclase,which increases the concentration of cGMP,and the latter messenger stimulates the dephosphorylation of myosin light chains(see Figure 2)and relaxation of the smooth muscle.Thus,any drug that increases cGMP might be of value in erectile dysfunction if normal innervation is present.Sildenafil(Viagra)acts to increase cGMP by inhibiting its breakdown by phosphodiesterase isoform 5.The drug has been very successful in the marketplace because it can be taken orally. However,sildenafil is of little or no value in men with loss of potency due to cord injury or other damage to innervation and in men lacking libido.Furthermore, sildenafil potentiates the action of nitrates used for angina,and severe hypotension and a few myocardial infarctions have been reported in men taking both drugs.It is recommended that at least 6 hours pass between use of a nitrate and the ingestion of sildenafil.Sildenafil also has effects on color vision,causing difficulty in blue-green discrimination.Two similar PDE-5 inhibitors,tadalafil and vardenafil,are available. The drug most commonly used in patients who do not respond to sildenafil is alprostadil,a PGE analog that can be injected directly into the cavernosa or placed in the urethra as a minisuppository,from which it diffuses into the cavernosal tissue. Phentolamine can be used by injection into the cavernosa.These drugs will cause erection in most men who do not respond to sildenafil. 99 mixtures containing lead, arsenic, or strychnine. Substances currently favored by practitioners of herbal medicine include ginseng and kava. Scientific studies of the process have shown that erection requires relaxation of the nonvascular smooth muscle of the corpora cavernosa. This relaxation permits inflow of blood at nearly arterial pressure into the sinuses of the cavernosa, and it is the pressure of the blood that causes erection. Physiologic erection occurs in response to the release of nitric oxide from nonadrenergic-noncholinergic nerves associated with parasympathetic discharge. Thus, parasympathetic innervation must be intact and nitric oxide synthesis must be active. (It appears that a similar process occurs in female erectile tissues.) Certain other smooth muscle relaxants eg, PGE1 analogs or antagonists if present in high enough concentration, can independently cause sufficient cavernosal relaxation to result in erection. As noted in the text, nitric oxide activates guanylyl cyclase, which increases the concentration of cGMP, and the latter messenger stimulates the dephosphorylation of myosin light chains (see Figure 2) and relaxation of the smooth muscle. Thus, any drug that increases cGMP might be of value in erectile dysfunction if normal innervation is present. Sildenafil (Viagra) acts to increase cGMP by inhibiting its breakdown by phosphodiesterase isoform 5. The drug has been very successful in the marketplace because it can be taken orally. However, sildenafil is of little or no value in men with loss of potency due to cord injury or other damage to innervation and in men lacking libido. Furthermore, sildenafil potentiates the action of nitrates used for angina, and severe hypotension and a few myocardial infarctions have been reported in men taking both drugs. It is recommended that at least 6 hours pass between use of a nitrate and the ingestion of sildenafil. Sildenafil also has effects on color vision, causing difficulty in blue-green discrimination. Two similar PDE-5 inhibitors, tadalafil and vardenafil, are available. The drug most commonly used in patients who do not respond to sildenafil is alprostadil, a PGE1 analog that can be injected directly into the cavernosa or placed in the urethra as a minisuppository, from which it diffuses into the cavernosal tissue. Phentolamine can be used by injection into the cavernosa. These drugs will cause erection in most men who do not respond to sildenafil