Other warning signs of an unsafe online pharmacy include outdated medication, no customer service, or suspiciously low prices. VIAGRA was administered to more than 3, patients aged 19 to 87 years, with ED of various etiologies organic, psychogenic, mixed with a mean duration of 5 years. Based upon measurements of sildenafil in semen of healthy volunteers 90 minutes after dosing, less than 0. In these studies, involving about patients, analyses of patient diaries showed no effect of VIAGRA on rates of attempted intercourse about 2 per week , but there was clear treatment-related improvement in sexual function: per patient weekly success rates averaged 1. The pharmacokinetics of sildenafil are dose-proportional over the recommended dose range. Cialis Daily tadalafil : This low-dose Cialis works by increasing blood flow to the penis over time, making it easier to have an erection when aroused.Sildenafil (Viagra). Tadalafil (Cialis). Vardenafil. These medicines are called PDE5 inhibitors. They enhance the effects of a chemical the body makes that relaxes . Viagra (sildenafil): As the first FDA-approved PDE5 inhibitor for ED, sildenafil is taken 30 minutes to four hours before sexual intercourse to help achieve . Viagra comes as tablets that you take by mouth, when needed. Available strengths: 25 mg, 50 mg, mg. Viagra comes in these strengths: 25 milligram (mg), .