How oral medicines differ Each oral medicine for ED has a slightly different chemical makeup. Brand names Why is this medication prescribed? These are commonly prescribed for an enlarged prostate or male pattern baldness. Generic versions of sildenafil, vardenafil and tadalafil are available. This medication is taken with or without food an hour before sex. Do not take a double dose to make up for a missed one.Before taking Viagra, be sure to tell your doctor about all medications you take, including prescription and over-the-counter types. Also describe any vitamins, herbs, or . The recommended dosage of Viagra for erectile dysfunction (ED) is 50 mg, taken as needed about 1 hour before sexual activity. You can take it 30 minutes to 4 hours before . generic Revatio and Viagra Used for Pulmonary Hypertension and Erectile Dysfunction MORE expand_more Check your prescription Prescription 20mg sildenafil (30 tablets) .