Side effects Side effects of oral ED medicines are often mild. When oral medications might not be safe Check with your health care team before you take any medicine for erectile dysfunction. Sexual activity may be a strain on your heart, especially if you have heart disease. Sildenafil works on the PDE5 enzyme in the lungs to relax the blood vessels. Ask your pharmacist or doctor for a copy of the manufacturer’s information for the patient. Brand names Why is this medication prescribed? Before starting, stopping, or changing any medications while taking sildenafil, please get the advice of your doctor or pharmacist.Viagra comes as an oral tablet in different dosages. Discuss with your doctor what’s an appropriate dose for you. For daily use, Goldstein said, the generic form, sildenafil, comes in mg tablets.  · You can take the drug 30 minutes to 4 hours before you anticipate sexual activity. Based on how well you respond to this dosage and any side effects you have, your doctor may either increase your. 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 sex — but shouldn’t be.