This medicine also works best when you take it an hour before sex on an empty stomach. 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. If you are taking sildenafil for PAH, take the missed dose as soon as you remember it. Medicines that block an enzyme called CYP3A4. When choosing one, tell your doctor or other health care professional what your preferences are, such as cost, ease of use, how long the drug’s effects last and side effects. The following nonprescription or herbal products may interact with sildenafil: cimetidine Tagamet HB , St.What to do in case you take too much Viagra Call your doctor if you think you’ve taken too much Viagra. You can also call to reach the American Association of Poison Control Centers. VIAGRA can cause your blood pressure to drop suddenly to an unsafe level if taken with certain other medicines. Do not take VIAGRA if you take any other medicines called “nitrates.” Nitrates are used to treat chest pain (angina). A sudden drop in blood pressure can cause you to feel dizzy, faint, or have a heart attack or stroke. Next steps Viagra (sildenafil) is a brand-name oral tablet prescribed for erectile dysfunction. The cost of the drug with and without insurance can depend on several factors, such as .