It's usually the first sign that the drug is taking effect. of these rights holders. This article has been viewed 296,295 times. Sildenafil citrate, sold as Viagra, Revatio and under various other trade names, became available in 1998. By using our site, you agree to our A. and Kell, P. (2009). Inhibitory effect of alcohol on cyclic GMP accumulation in human platelets. It’s not safe to take another one before the 24-hour mark, even if you aren’t feeling the effects any more, because there will still be some Viagra in your system and so you could raise the amount of Viagra in your body above a safe level.Even if you’re taking less than the maximum daily dose of Viagra (100mg), you still shouldn’t take extra Viagra on the same day because you need a doctor to check a higher dose is safe for you first. Epidemiology of erectile dysfunction. A randomized placebo-controlled study. Once you take it, it should start working for you in about 30 mins. Dr Kathryn Basford is a qualified GP who works as a GP in London, as well as with Zava. To get the best results with Viagra, take the pill on an empty stomach. In the US, either ask your family doctor or go to a clinic.