Dosing The dose of this medicine will be different for different patients. How should this medicine be used? A newer form of the medicine that dissolves on the tongue might work faster than the pill that you swallow. The following nonprescription or herbal products may interact with sildenafil: cimetidine Tagamet HB , St. Use only the brand of this medicine that your doctor prescribed.The four main medicines taken by mouth for erectile dysfunction are: Avanafil (Stendra). Sildenafil (Viagra). Tadalafil (Cialis). Vardenafil. These medicines are called .  · For oral dosage forms (suspension or tablets): Adults—20 milligrams (mg) 3 times a day. Your doctor may adjust your dose as needed and tolerated. . Last updated on Uses Warnings Before taking Dosage Side effects Interactions FAQ What is sildenafil? Sildenafil relaxes muscles of the blood vessels and .