I'd give it back. I don't like cocaine or what it does to people at all (something I've had a great deal of experience of- I've lived with a cokehead), but it's none of my business if people choose to take it. (I'll admit I'd be sorely tempted to "accidentally" spill it everywhere, but I wouldn't do it) I would however draw attention to the fact that I knew what it was, and ask the patient how much they had taken, and if they'd taken anything else as well.