Check with your doctor immediately if any of the following side effects occur: Less common burning feeling in the chest or stomach burning, crawling, itching, numbness, prickling, “pins and needles”, or tingling feelings cloudy or bloody urine. Severe liver disease. How should this medicine be used? Sildenafil Viagra. Be sure to let your doctor and pharmacist know that you are taking these medications before you start taking sildenafil. 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.Find patient medical information for sildenafil oral on MD including its uses, side effects and safety, interactions, pictures, warnings and user ratings. Strengths: 25 mg, 50 mg, mg; Brand: Viagra. Form: oral tablet; Strengths: 25 mg, 50 mg, mg; Adult dosage (ages 18–64 years) Typical dosage: 50 mg, taken as needed about 1 hour before. Sildenafil (Viagra) is used to treat erectile dysfunction (impotence; inability to get or keep an erection) in men. 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 .