Viagra is the trade name, the generic name is sildenafil citrate, the first time to take 25mg whether it is effective depends on the individual, the general recommended dose of 50mg. Sildenafil citrate is a cyclic guanosine monophosphate specific type 5 phosphodiesterase inhibitor, when there is sexual stimulation caused by local nitric oxide release, sildenafil can promote cavernous smooth muscle relaxation, promote penile erection. Whether or not the first dose of sildenafil citrate 25mg has an effect depends on the individual, the generally recommended dose is 50mg/dose, up to once a day, and can be taken up to 1 hour prior to sexual activity as needed. The dose may be increased to a maximum of 100mg or decreased to 25mg depending on individual potency and tolerance. Sildenafil citrate should not be combined with nitrates or guanylate cyclase agonists such as Leoxicidin to avoid hypotension. Adverse reactions such as headache, flushing, dyspepsia, abnormal vision and myalgia may occur after use.