You can take this medicine with or without food 30 minutes before sex, depending on the dose. However, if it is almost time for the next dose, skip the missed dose and continue your regular dosing schedule. Sildenafil is in a class of medications called phosphodiesterase PDE inhibitors. Its effects might last longer if you have mild to moderate ED. Do not start these medications while taking sildenafil without discussing it with your healthcare provider. Do not stop taking sildenafil without talking to your doctor. What side effects can this medication cause?Sildenafil (Viagra). This medicine works best when you take it on an empty stomach an hour before sex. The body takes longer to absorb it after a high-fat meal or . Since , Viagra (sildenafil) has been used for treating erectile dysfunction (ED). It’s available by prescription for males ages 18 years and older. Side . Sildenafil (Viagra) is used to treat erectile dysfunction (impotence; inability to get or keep an erection) in men. Sildenafil (Liqrev, Revatio) is used to improve the ability to exercise in .