For optimal results, Viagra or generic sildenafil should be taken about one hour in advance of sex. Children—Use is not recommended. A normal dose of Viagra should remain effective for approximately four hours. Other medications used to treat high blood pressure and heart disease, such as alpha blockers, can interact with Viagra and cause issues such as fainting. If you have erectile dysfunction, the enzyme phosphodiesterase type 5 PDE5 often works too quickly to make your penis flaccid, which can make it difficult for you to get an erection in the first place or keep one for long enough to ejaculate.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, BCGP.  · 3 Answers. Viagra works in response to sexual stimulation to increase the blood flow to the penis leading to an erection. Viagra does not result in an erection without sexual stimulation. When a man is aroused, muscles in the penis relax to allow this greater blood flow. Viagra helps to elevate the levels of a substance (cyclic guanosine. Dosage. The dose of Cialis you need will depend on whether you’re taking it daily or as needed. Daily doses are often between and 5 milligrams (mg). The as-needed dose is usually 10 mg but can range from 5–20 mg. The typicaldose of Viagra is 50 mg. However, the dosage can range from 25– mg.