Your doctor may adjust your dose as needed and tolerated. One study found that a firm erection could be achieved with as little as 12 minutes of stimulation, but for some men it can take up to an hour. Slowly pull back the plunger until the required dose. Use only the brand of this medicine that your doctor prescribed. However, it stays in your body for up to 36 hours and can be taken every 24 hours, so it can work for you constantly. These medicines are called PDE5 inhibitors.Description and Brand Names Drug information provided by: Merative, Micromedex ® US Brand Name Revatio Viagra Descriptions Sildenafil is used to treat men who have . Oral medicines for ED usually work well and cause few side effects. The four main medicines taken by mouth for erectile dysfunction are: Avanafil (Stendra). Sildenafil . For treatment of erectile dysfunction: For oral dosage form (tablets): Adults up to 65 years of age—50 milligrams (mg) as a single dose no more than once a day, 1 hour .