Viagra generally refers to sildenafil, the strength of the drug can usually be maintained for about 4 to 6 hours, but it does not mean that the patient can continue to have an erection for 4 to 6 hours after taking it, and it is necessary to have an erection after taking sildenafil through sexual stimulation. Sildenafil is a PDE5 inhibitor class of drugs, for the treatment of male erectile dysfunction has a certain therapeutic effect, usually in the sexual life of 0.5 ~ 4 hours before taking as needed, after taking the need to receive sexual stimulation to take effect. It usually lasts for 4~6 hours. It is important to note that sildenafil should not be taken at the same time as any form of nitrates, which may result in hypotension, which can be life-threatening in severe cases. Some patients of Sildenafil may experience adverse reactions such as dizziness, dizziness, loss of vision, and flushing after taking it. If you need to use the drug need to be standardized use under the guidance of a doctor, the emergence of discomfort serious should seek medical attention in a timely manner.