You need to be sexually excited for it to work. Sildenafil may cause side effects. Side effects Side effects of oral ED medicines are often mild. Oral medicines for ED usually work well and cause few side effects. Take sildenafil as needed before sexual activity. If you buy sildenafil for erectile dysfunction, it can also be known by the brand names Viagra, Aronix, Liberize and Nipatra. Continue to take sildenafil even if you feel well.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.  · Maintenance: 25 to mg orally once a day, as needed, 1 hour prior to sexual activity Comments: This drug may be taken anywhere from 30 minutes to 4 hours before sexual activity. Use: Treatment of erectile dysfunction. Usual Adult Dose for Pulmonary Hypertension. Revatio (R): Oral: Initial dose: 5 or 20 mg orally three times a . SILDENAFIL (sil DEN a fil) is used to treat erection problems in men. This medicine may be used for other purposes; ask your health care provider or pharmacist if you have .