Sorry something went wrong with your subscription Please, try again in a couple of minutes Retry. You may opt-out of email communications at any time by clicking on the unsubscribe link in the e-mail. This site is intended only for U. Sildenafil works on the PDE5 enzyme in the lungs to relax the blood vessels. Alpha-blockers are sometimes prescribed for prostate problems or high blood pressure.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.  · 50 mg mg Typical dosages The following information describes commonly recommended dosages for ED. However, be sure to take the dosage your doctor prescribes for you. Your doctor will. Dizziness, headache, flushing, or stomach upset may occur. Vision changes such as increased sensitivity to light, blurred vision, or trouble telling blue and green colors apart may also occur. If.