Do not stop taking sildenafil without talking to your doctor. Oral medicine for ED also might not be safe or work well if you have: Very low blood pressure, called hypotension, or uncontrolled high blood pressure, called hypertension. You should only be treated with one of these products at a time. You can take it in a small dose daily or in a larger dose as needed. Sildenafil treats PAH by relaxing the blood vessels in the lungs to allow blood to flow easily.Viagra works by increasing blood flow to your penis, which helps you have and keep an erection. But this drug only works if you’re sexually aroused. Viagra comes as tablets . To treat erectile dysfunction-ED, take this drug by mouth as directed by your doctor, usually as needed. Take sildenafil at least 30 minutes, but no more than 4 hours, before . Viagra or generic sildenafil for ED should not be used in women. For seniors: The kidneys, liver, or heart of older adults may not work as well as they used to. This can cause your .