The generic name of Viagra is sildenafil citrate, and the lack of effect after taking it may be due to lack of sexual stimulation, taking the medication at the wrong time, or insufficient dosage of the medication. You should take a suitable dose of sildenafil citrate orally 30 minutes – 1 hour before intercourse, and you must have sexual stimulation after eating so that it can work best. 1. There must be sexual stimulation: after taking sildenafil citrate, it also needs to be under certain sexual stimulation to work. 2. Master the time of medication: generally 30 minutes to 1 hour before sex, older patients, due to the slow absorption of drugs, need to be taken 1 hour in advance. 3. Master the dose: the recommended dose of sildenafil citrate for the first application is 50 mg, 50 mg is not enough can be increased to 100 mg. Sildenafil citrate may cause facial edema, shock, weakness, pain, angina, dry mouth, vomiting and other symptoms after taking, about 24 hours gradually return to normal. If there is a sudden loss of vision, or hearing loss, the drug should be stopped immediately and medical attention sought as soon as possible. The use of sildenafil is contraindicated in patients taking any dosage form of nitrates, either regularly or intermittently. Sildenafil is contraindicated in patients who are hypersensitive to any of the ingredients in sildenafil. Sildenafil Citrate is a prescription drug and is recommended to be taken as prescribed by a physician.