These include itraconazole Sporanox, Tolsura , ketoconazole Extina, Ketozole, others and ritonavir Norvir. Often, these are prescribed for chest pain called angina. They’re often the first line of treatment for trouble getting or keeping an erection, called erectile dysfunction ED. This is a type of high blood pressure that occurs between the heart and the lungs. Sildenafil continues to work for up to 4 hours, although its effectiveness decreases after 2 hours. When oral medications might not be safe Check with your health care team before you take any medicine for erectile dysfunction.Dosage for generic Viagra. Generic Viagra (sildenafil) is available as a , , , and milligram (mg) pill and in liquid form. It’s meant to be taken as needed — about an hour before. Maria Hudson. 4 hrs. ago. Anna Nelson. 6 hrs. ago. David Muldon. 8 hrs. ago.  · Brand names: Revatio, Viagra Drug classes: Agents for pulmonary hypertension, Impotence agents. Medically reviewed by Kaci Durbin, MD. Last updated on Uses; Warnings; Before taking; Dosage; Side effects; Interactions; FAQ; What is sildenafil? Sildenafil relaxes muscles of the blood vessels and increases blood flow to .