Severe liver disease. Alpha-blockers are sometimes prescribed for prostate problems or high blood pressure. Priapism, or an erection lasting longer than four hours, is also a potential serious side effect. Which One is Safer? This medication is taken with or without food an hour before sex. Studies show that Tadalafil and Sildenafil are both effective in about sixty to eighty percent of men with ED — depending on the dosage. These side effects are generally not dangerous, but consult your doctor if they persist or are particularly bothersome.Drug Images What is Viagra? If you have erectile dysfunction (ED), your doctor may recommend that you take Viagra. It’s a prescription drug that’s used in adult males* with ED. With ED, you. Typical dosing for Sildenafil (Viagra) The typical dose is to take 50 mg of sildenafil (Viagra) by mouth, about 1 hour before sex, as needed. The maximum recommended dose is mg. Don’t take sildenafil (Viagra) more than once daily (at any dose). Written by Brian Leonard, PharmD, BCACP, test1.rug: usa. It really depends based on the person. Most of the time, Viagra starts to work after 30 minutes. Some people note that Viagra works faster when taken on an empty stomach. The effects of Viagra usually last around 4 to 5 hours, in which time it is easier to have an erection. But in some cases, this window of time See more.