Viagra generally refers to sildenafil and can be used by men over the age of 18 with erectile dysfunction. Sildenafil is a selective type 5 phosphodiesterase inhibitor, which can inhibit type 5 phosphodiesterase on vascular smooth muscle, with vasodilating effect, thus increasing penile blood supply to make penile erection, and male patients over 18 years old with erectile dysfunction have an improved effect after oral intake. Sildenafil is contraindicated in children and women, elderly over 65 years old should reduce the dose of the drug. The drug is contraindicated in patients who are allergic to sildenafil, severe hepatic impairment, unstable angina, recent myocardial infarction or stroke. And the drug is contraindicated in combination with nitrates, otherwise severe hypotension can occur. Adverse effects of sildenafil include flushing, palpitations, angina, asthenia, dyspnea, myalgia, myalgia, increased nocturia, dizziness, headache, drowsiness, irritability, nausea, vomiting, and abdominal pain. Men over 18 years old can use sildenafil as prescribed by the doctor, and should go to the hospital in time if any related adverse reactions occur after taking the drug.