Viagra, also known as sildenafil citrate, is sold in China under the trade name Viagra. The way Viagra is used, it should be taken half an hour before the start of sex, and it has the effect of helping to improve erectile function within 4 hours, that is, the time limit of the effect is generally within 4 hours, and more than 4 hours, the effect of the drug may disappear. Therefore, for the use of Viagra treatment, generally can improve erectile function within 4 hours. Now a new generation of phosphodiesterase inhibitors, such as tadalafil, which is used more often now, the effective time can be extended to 36 hours, also taken half an hour before the start of sex, the effect can be maintained for 36 hours, so its effective time is much longer, and is currently a better alternative to Viagra.