fly in the face of: o que isso significa?

So imagine you have this friend whose habit is taking medications for every little part of his body. One day, you tell him he's overdoing it and that should get him into trouble sometimes in the future. He said you're right, but that he just can't stop it since he started to take more pills than what was prescribed and now it seems he is kind of addicted to the pills. He can't sleep without them.

When he says that, you get very worried and say: That kind of attitude flies in the face of any doctor's recommendations. You should only do what he prescribes you. When something flies in the face of something or someone, it means it goes against it, it challenges it. So imagine a new discovery in science flies in the face of everything they'd believed in till that point. That's it for today guys. Take care!