Oral ED medicines might not work or might be dangerous if you also take any of the following: Nitrate medicines. A newer form of the medicine that dissolves on the tongue might work faster than the pill that you swallow. Sildenafil usually should not be taken more than once every 24 hours. What should I do if I forget a dose? Do not stop taking sildenafil without talking to your doctor. These include itraconazole Sporanox, Tolsura , ketoconazole Extina, Ketozole, others and ritonavir Norvir. Some medications should not be taken with sildenafil.What is Viagra? If you have erectile dysfunction (ED), your doctor may recommend that you take Viagra. It’s a prescription drug that’s used in adult males* with ED. With ED, you have trouble.  · 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. Sildenafil (Viagra, Revatio) is a moderately priced drug used to treat erection problems in men. It is also used to treat pulmonary arterial hypertension, a serious heart and lung drug is more popular than comparable drugs. It is available in generic and brand versions. Generic sildenafil is covered by most Medicare and insurance plans, but .