Viagra, known as sildenafil in clinical medicine, is not a hormone drug, it is a kind of drug for treating erectile dysfunction in men and belongs to the class of PDE5 inhibitors. Sildenafil can improve blood flow to the penis and promote the release of nitric oxide in the corpus cavernosum of the penis, thus improving the hardness of the penis erection. Sildenafil is a short-acting drug that usually takes effect about 30 to 60 minutes after taking it, and its effects last for about 4 to 6 hours. The main common adverse effects of this drug are headache, flushing, dizziness, nasal congestion, indigestion, and visual abnormalities. This drug is contraindicated in women, people who are allergic to the product, children, people whose recent cardiovascular state does not allow them to have sex. It should be noted that sildenafil belongs to the prescription drugs, should follow the doctor’s instructions to take treatment, do not blindly take on their own to avoid harm to the body, in addition, sildenafil can not be taken with any nitrate drugs, there may be life-threatening.