Inside United States borders Viagra requires an Rx from your MD. The problem is that it does have side effects and is not effective in everyone who uses it. By the sound of this you want it for recreational purposes. I am not against that, but, the problem is that like as has been said elsewhere it does not really change your erection in any manner from what you would have if simply aroused.
Some years ago I used it. The way I knew that it was kicking in was that my vision had a color spectrum change in that things took on a bluish tint. I however did legitimately suffer from ED and for me it was hit and miss unless I did some modification to what one was supposed to do.
In the US there are three orals currently available. Viagra (Sildenafil) Levitra (Vardenafil) and Cialis which has the advantage of being longer acting, but not much else. Vardenafil is claimed to be more effective by it's manufacturer.
In my case I finally decided that enough was enough with the hit and miss effectiveness and paid out of pocket to have my ED evaluated and go for the injectable medication. I have never regretted it. If your problem is legitimate ED forget the orals and go directly for the injectable. It works and it works well every time. The pills are actually more expensive and have mixed results.