Please help! what movie is this!?

ok so one night 3 or 4 years ago i was watching tv late one night and there was this kick ass movies about a bunch of kids/teens/young people what ever. and i *think* they were testing this virtual reality game. I cant rememeber many details at all. i think some people in the game got killed and the main character may have been posioned. Also there was like a MASSIVE twist at the end and they werent sure if they were still in the game or not and they kill people in what is potentially the real world but u never know for sure. (or something like that)

this isnt it is it?

Its driven me crazy that i have no idea what movie this is, i didnt realize how much i liked it when i was watching it at the time.