If you experience chest pain, dizziness, or nausea during sexual activity, call your doctor immediately and avoid sexual activity until your doctor tells you otherwise. However, if you take sildenafil with a high-fat meal, it will take longer for the medication to start to work. Before starting, stopping, or changing any medications while taking sildenafil, please get the advice of your doctor or pharmacist. Sildenafil is in a class of medications called phosphodiesterase PDE inhibitors. Erectile dysfunction is a condition where the penis does not harden and expand when a man is sexually excited, or when he cannot keep an erection. Remove the cap by pushing it down and twisting it counter-clockwise. · Adults 65 years of age and older—25 mg as a single dose no more than once a day, 1 hour before sexual intercourse. Alternatively, the medicine may be taken 30 .  · Sildenafil is used to treat men who have erectile dysfunction (also called sexual impotence). Sildenafil belongs to a group of medicines called phosphodiesterase . Products and services Erectile dysfunction: Viagra and other oral medications By Mayo Clinic Staff Medicines that you take by mouth are called oral .