Viagra, also known as sildenafil, is a drug used to treat and improve erectile dysfunction in men, and is generally recommended to be taken one hour before sex to allow the drug to work better. After taking sildenafil orally, there is usually a peak in blood concentration within half an hour to two hours, and the duration is about four hours. The therapeutic effect can be achieved within four hours, but the effect is usually stronger in the first two hours than in the later hours. The degree of absorption may vary as each person’s constitution is different. It is not recommended for people with heart disease, as it can lead to a recurrence of heart disease. It is best not to take antihypertensive drugs together with Viagra-like drugs, and always use them under the guidance of a doctor.