It lasts 4 to 5 hours, or longer if you have mild to moderate ED. However, if you take sildenafil with a high-fat meal, it will take longer for the medication to start to work. Your doctor will tell you not to take sildenafil if you are taking street drugs containing nitrates. When choosing one, tell your doctor or other health care professional what your preferences are, such as cost, ease of use, how long the drug’s effects last and side effects. Oral medicines for ED usually work well and cause few side effects. If you are taking sildenafil to treat PAH, you will probably take sildenafil three times a day with or without food.Regardless of your age, you’ll take your prescribed dose about an hour before sex. If the prescribed dosage of Viagra works well for you and doesn’t cause bothersome side . Still, if your doctor says it’s okay for you to use and you end up paying out-of-pocket, you can still get the generic for a lot less than Viagra. According to price-comparison site. So you do not need to take this medication on a daily schedule. If you forgot to take a dose before having sex, take it as soon as you remember. Will I need to use Viagra long term?