It is used as a treatment for adult men with erectile dysfunction, sometimes known as impotence. This is when a man cannot get, or keep a hard, erect penis suitable for sexual activity. Viagra usually works in around 30 minutes to an hour, but this is different for every man. It is recommended to take it an hour before sex, but it can work up to 4 hours after. It may take longer to work if you have eaten a big meal before taking Viagra.