Viagra is the common name for Sildenafil, and one Sildenafil usually lasts for about 4 hours or so. Sildenafil is a drug used to treat erectile dysfunction in men. According to the drug’s instructions, sildenafil is rapidly absorbed after oral intake, with blood concentrations peaking at 30 to 120 minutes for the best results when taken orally on an empty stomach, and 90 to 180 minutes when taken after a meal. Sildenafil has a systemic clearance of 41 L/h and a terminal half-life of about 4 hours and therefore a duration of about 4 hours. However, according to the pharmacokinetic curve of sildenafil, sildenafil has the best effect in the first two hours after taking it, while the blood concentration gradually decreases in the next two hours, with the second effect, thus it is more suitable to take it about 1h before the need for sexual activity. Sildenafil is mainly metabolized by the liver microsomal enzymes cytochrome P4503A4 and cytochrome P4502C9, for the lower activity of these two enzymes, the elderly over 65 years of age, or people with liver or kidney impairment, sildenafil metabolism in the blood is slower, and the drug lasts longer. Consult your physician and reduce the dosage appropriately. Common adverse reactions to sildenafil include headache, flushing, dizziness, nasal congestion, indigestion and visual abnormalities. The drug is prohibited in combination with nitrate drugs, and is contraindicated in women, people who are allergic to the product, children, and people whose recent cardiovascular status makes them unfit for sexual intercourse. Sildenafil should be used as prescribed by a doctor, do not use the drug without authorization.