Sildenafil：mechanism of action,side effects,precaution
Sildenafil, sold as the brand name Viagra among others, is a medication used to treat erectile dysfunction and pulmonary arterial hypertension.It is unclear if it is effective for treating sexual dysfunction in women.It is taken by mouth or injection into a vein.Onset is typically within 20 minutes and lasts for about 2 hours.
Mechanism of action
By selectively inhibiting phosphodiesterase type 5, it allows corpus cavernosum smooth muscle to relax, potentiating erections during sexual stimulation. Blood pressure is reduced transiently by sildenafil, but more marked hypotension may occur during concurrent administration of sildenafil and organic nitrates; this combination is contraindicated. Sildenafil is rapidly absorbed, with dose-proportional peak plasma concentrations within 1 hour of administration. The elimination half-life is 3 to 5 hours. Dosages usually begin at 50mg taken when needed =1 hour before sexual activity no more than once daily. The maximum dose is 100mg when needed once daily and lower doses (e.g. 25mg) may be used in elderly patients and those with hepatic or renal impairment or receiving cytochrome P450 enzyme CYP3A4 inhibitors, such as ritonavir, saquinavir, ketoconazole, erythromycin or cimetidine.
More than 3000 patients with erectile dysfunction of organic (e.g. diabetes or spinal cord injury), psychogenic or mixed origin received sildenafil 5 to 100mg or placebo in fixed- or titrated-dose trials. Sildenafil was associated with dose-related improvements in the frequency, hardness and duration of erections and in patients' abilities to achieve and maintain erections adequate for successful sexual intercourse. In titrated-dose trials, the most commonly effective doses were 50 or 100mg, although lower doses were effective in some patients. Sildenafil was significantly more effective than placebo in erectile dysfunction of all tested causes. The efficacy of sildenafil was not affected by patient age (> or < or =65 years) or by antihypertensive or antidepressant medications. The drug was effective in patients with severe erectile dysfunction. Efficacy was maintained in long term (1-year) studies. Sildenafil also appears to improve the quality of life of both patients and their sexual partners.
Side effects
Get emergency medical help if you have signs of an allergic reaction to sildenafil: hives; difficulty breathing; swelling of your face, lips, tongue, or throat.
Precaution
Stop taking sildenafil and get emergency medical help if you have:
heart attack symptoms - chest pain or pressure, pain spreading to your jaw or shoulder, nausea, sweating;
vision changes or sudden vision loss;
erection is painful or lasts longer than 4 hours (prolonged erection can damage the penis);
ringing in your ears, or sudden hearing loss;
irregular heartbeat;
swelling in your hands, ankles, or feet;
shortness of breath;
seizure (convulsions); or
a light-headed feeling, like you might pass out.
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