Sildenafil is in a class of medications called phosphodiesterase PDE inhibitors. Do not start these medications while taking sildenafil without discussing it with your healthcare provider. Before starting, stopping, or changing any medications while taking sildenafil, please get the advice of your doctor or pharmacist. Medicines that block an enzyme called CYP3A4. John’s wort. Talk to your doctor about eating grapefruit and drinking grapefruit juice while taking this medicine. The best time to take sildenafil is about 1 hour before sexual activity, but you can take the medication any time from 4 hours to 30 minutes before sexual activity.Sildenafil is in a class of medications called phosphodiesterase (PDE) inhibitors. Sildenafil treats erectile dysfunction by increasing blood flow to the penis during sexual . Products and services Erectile dysfunction: Viagra and other oral medications By Mayo Clinic Staff Medicines that you take by mouth are called oral . Sildenafil oral tablet is a prescription medication used to treat erectile dysfunction (ED) and pulmonary arterial hypertension (PAH). Sildenafil can be taken .