Viagra generally refers to sildenafil citrate tablets, after eating will not take effect immediately, under normal circumstances after eating sildenafil citrate tablets about half an hour after the effect, the physical condition is different, the time of effect will have some difference. Sildenafil citrate tablets are selective phosphodiesterase type 5 inhibitors. It usually needs to be taken one hour before the start of sex. After taking sildenafil citrate tablets, it is effective in improving erectile dysfunction caused by various reasons. The drug takes effect in about half an hour, and after taking sildenafil citrate tablets, penile erection occurs only with sexual stimulation. When taking sildenafil citrate tablets, you should pay attention to the fact that the drug should not be taken frequently for a long period of time, so as not to cause the body to have adverse reactions, such as flushing, headache, dizziness and so on. In addition, sildenafil is forbidden to be combined with nitrate drugs, and the drug is forbidden for women, people who are allergic to the product, children, and people whose recent cardiovascular state is not suitable for sexual life. When taking the drug, if there is an adverse reaction, you should go to the hospital in time, and under the guidance of the doctor to carry out appropriate treatment.