I have a patient who is a liar. He is telling me a story to get pain medications. I don't have any proof he is a liar. He has no incriminating evidence that I can find. There is no labwork that I can do to prove his pain one way or the other. Without evidence, I am inclined to give him the pain meds. I am willing to play this game. All he must do is play the game with me.
Instead, the staff catch him with secret pain meds not prescribed by me, and eating meals from the cafeteria when he swears that he can't keep anything down. All he has to do is play along, and even that, he can't do.
And that is the sad fact about addicts. He has me beat, and still, he cannot stop himself from ruining it.