Viagra doesn't work that way, you need to take it 30-60 mins before sex and it can last several hours. So unless you are dead certain about getting some action you'll just be wasting it. And there are some funny side effects (some people can't get it down after lol, they actually show that in one of the American Pie movies). There's also that nasty part about not taking it with nitrates...unless you want to die a very painful death.
Millions per month - amazing. I wouldn't have imagined the sheer amount viagra companies are earning.