Viagra is the trade name, generic name sildenafil, there is no professional data to show the duration of efficacy of the drug after use, but from the drug metabolism time, half an hour to four hours before coitus, there are effects. Sildenafil is a type 5 phosphodiesterase inhibitor and is currently used to treat erectile dysfunction in men, improving penile erection and maintaining better erection hardness. From the point of view of drug metabolism level, about 2 hours after the use of the drug, to reach the peak of blood concentration, followed by blood concentration will slowly decay. Therefore, for patients who really need to take the drug, it is recommended to use the drug about half an hour to four hours before intercourse, in this period of time are generally able to play the effect of the drug. In addition, sexual stimulation should be given before intercourse after taking the drug. The main common adverse effects of sildenafil tablets include headache, flushing, dizziness, nasal congestion, indigestion, and visual abnormalities. The drug is forbidden to be used in combination with nitrate drugs, it is contraindicated for women, people who are allergic to the product, children, and people whose recent cardiovascular state makes them unfit for sexual intercourse. The drug is a prescription drug and should be used in accordance with medical advice.