Viagra generally refers to sildenafil, starting at 50mg for the first time and taking up to 100mg at a time. Some special people should start taking it from 25mg, such as people with liver or kidney insufficiency, and people over 65 years old. Sildenafil, a medication for erectile dysfunction, can improve erectile function symptoms and allow for increased sexual satisfaction. The first dose of sildenafil for the general population starts at 50mg and should not exceed 100mg per dose. Some special groups of people should start taking it from 25mg, such as people with liver or kidney insufficiency, and people over 65 years old. Read the instructions carefully before taking the medication and take it according to the dosage, as well as under the guidance of your doctor. The common adverse reactions of sildenafil mainly include headache, flushing, dizziness, nasal congestion, indigestion, visual abnormalities and so on. Sildenafil is forbidden to be used in combination with nitrate drugs, and the drug is contraindicated for women, people who are allergic to the product, children, and people whose recent cardiovascular status makes them unfit for sexual intercourse. The drug should be used under the supervision of a doctor.