Fenton's Party

When I first saw this I assumed that Fenton gave Serena a date rape drug and told her it was cocaine. But I just saw this again and noticed just before they went in his room, they were up on the roof and he gave her a pill. Which was the date rape drug? I am confused, had she already taken a Mickey Finn when he pulled out the white powder? Or was that the date rape drug? How would he explain not taking any himself, it must have worked really fast or she would get suspicious.

Not withstanding the date rape, Fenton did have pretty good taste, did you see how fancy that place was decorated with the bar, the lighting, layout, music and all. It was a shame he turned out to be such a creep because it looked like we were finally going to have an interesting character who had his act together instead of losers like that Tommy and the bum who lived in a cellar. I guess that was the point of the movie, Serena found out that all that glitters isn't gold and she was out of her league.