Elliot: How did you know that he was lying?
Dr. Cox: Well, you said that he wasn't; and as a rule, I always take whatever you say and just go in the exact opposite direction with it. But, more than that, Barbie, the main reason is because... well, lookit, medicine is all about experience. Hell, wouldja... wouldja like to go ahead and have a look at who the last doctor was to give Thompson drugs?
Elliot: You?
Dr. Cox: Oh!
Elliot: Why didn't you say something earlier?
Dr. Cox: What, give up my front-row seat to Barbie's Wild Ride? "He's a drug addict! He's not a drug addict! You... make... me... doubt... my...self! You're more interested in being right than doing what's right! And I just couldn't take it! Can't take it anymore!"