Generic name: sildenafil (oral) [ sil-DEN-a-fil ] Brand names: Revatio, Viagra Drug class: Impotence agents Medically reviewed by Sophia Entringer, PharmD. Last updated on Uses Warnings Before taking Dosage Side effects Interactions FAQ What is Viagra?Tadalafil Cialis. This can happen if you are sick with fever, diarrhea, or vomiting; sweat a lot; or do not drink enough liquids. This increased blood flow can cause an erection. The best time to take sildenafil is about 1 hour before sexual activity, but you can take the medication any time from 4 hours to 30 minutes before sexual activity. Common ones include: Flushing of the skin. Brian Staiger Pharm. Use the oral syringe provided with your medication to measure and take your dose.Sildenafil (Liqrev, Revatio) is used to improve the ability to exercise in adults (Liqrev, Revatio) and children 1 year of age and older (Revatio) with pulmonary arterial hypertension (PAH; high blood pressure in the vessels carrying blood to the lungs, causing shortness of breath, dizziness, and tiredness). Costs. Viagra is a brand-name drug, while sildenafil is a generic form of Viagra. Brand-name medications usually cost more than generics. The actual price you’ll pay for either drug depends. Viagra generic form Viagra is a brand-name medication. It’s also available in a generic form called sildenafil. How does Viagra work and how long does it last? You may have questions.Common ones include: Flushing of the skin. Sildenafil works on the PDE5 enzyme in the lungs to relax the blood vessels. When a man is sexually stimulated, his body’s normal response is to increase blood flow to his penis to produce an erection. If you have certain health conditions or are taking certain medications, your doctor may tell you to take sildenafil less often. It lasts 4 to 5 hours, or longer if you have mild to moderate ED. The body takes longer to absorb it after a high-fat meal or alcohol.