It works for up to 36 hours. This boosts blood flow and helps you get an erection from sexual activity. Side effects Side effects of oral ED medicines are often mild. They include dutasteride Avodart, Jalyn and finasteride Propecia, Proscar. How oral medicines differ Each oral medicine for ED has a slightly different chemical makeup.Sildenafil (Viagra, Revatio) is a moderately priced drug used to treat erection problems in is also used to treat pulmonary arterial hypertension, a serious heart and lung . Cialis is only available by prescription in the United States. Viagra: Additionally, a study from Trusted Source. found that combining L-arginine and sildenafil (Viagra) worked . Viagra is usually taken only when needed, 30 minutes to 1 hour before sexual activity. You may take it up to 4 hours before sexual activity. Other drugs may interact with .