Before starting, stopping, or changing any medications while taking sildenafil, please get the advice of your doctor or pharmacist. It works for up to 36 hours. Read and follow these instructions carefully before you start using sildenafil and each time you get a refill of your medicine. Ask your doctor if you have any questions. If you are taking sildenafil to treat PAH, you will probably take sildenafil three times a day with or without food. A newer form of the medicine that dissolves on the tongue might work faster than the pill that you swallow.Sildenafil under the name Viagra is used to treat erectile dysfunction (impotence) in men. Another brand of sildenafil is Revatio, which is used to treat pulmonary arterial . Viagra comes as an oral tablet that contains the active drug sildenafil. Sildenafil is also available in generic form. Viagra is classified as a phosphodiesterase type 5 (PDE5) . Sildenafil is used to treat male sexual function problems (impotence or erectile dysfunction-ED). In combination with sexual stimulation, sildenafil works by increasing blood flow to .