Viagra is the trade name, generic name is sildenafil citrate, 100 mg size of sildenafil, how many years to take depends on the shelf life of the drug, in the drug shelf life is safer to take. Generally 60 months. Sildenafil is used clinically for erectile dysfunction, it is a prescription drug and needs to be taken under the guidance of a doctor of choice. The usual dosage is 25, 50 or 100 milligrams, 100 milligrams of sildenafil is the maximum dosage, which is taken up to once a day, and the dosage also depends on the shelf life of the drug, sildenafil has a shelf life of 60 months, and needs to be taken within 60 months. Taking sildenafil may have certain adverse reactions, such as headache, nausea, indigestion, vision disorders, etc. If the side effects are more serious, seek medical attention. Sildenafil is forbidden to be used in combination with nitrate drugs, and this drug is contraindicated in women, people who are allergic to this product, children, and people whose recent cardiovascular state does not allow them to have sex.