If you are taking sildenafil to treat PAH, you will probably take sildenafil three times a day with or without food. This medicine also works best when you take it an hour before sex on an empty stomach. Do not take a double dose to make up for a missed one. You can take sildenafil with or without food. Some medications should not be taken with sildenafil. Kidney disease that needs to be treated with dialysis. These include itraconazole Sporanox, Tolsura , ketoconazole Extina, Ketozole, others and ritonavir Norvir.Sildenafil is in a class of medications called phosphodiesterase (PDE) inhibitors. Sildenafil treats erectile dysfunction by increasing blood flow to the penis during sexual stimulation. This increased blood flow can cause an erection. Sildenafil treats PAH by relaxing the blood vessels in the lungs to allow blood to flow easily. It’s a prescription drug that’s used in adult males* with ED. With ED, you have trouble getting or keeping an erection. To find out more about how Viagra is used for ED, see the “ What is.  · a light-headed feeling, like you might pass out. Common sildenafil side effects may include: flushing (warmth, redness, or tingly feeling); headache, dizziness; heartburn, nausea, or upset stomach. abnormal vision (blurred vision, changes in color vision) runny or stuffy nose, nosebleeds; sleep problems (insomnia); or.