If she was going to be staying put, I would drop it on the floor next to her and make sure she saw me do it. That way, I'm not holding it, I'm not handing it back to her, and she's not going to kick off and get violent at me for taking her stuff away (legality is all very well when the nearest copper is not, in fact, near. Given a choice between not getting beaten up, or, getting beaten up, and then dragging through the courts, I know what I choose). If she was being transported to hospital, though, then I would hang on to it and take it with me to give to the hospital staff so they can find out what exactly she's had. This would also be a bargaining point if she tried to insist on being taken in unnecessarily ("well, I could take you in, but I'd have to give them this because otherwise they can't treat you properly"). I can't honestly imagine that the police would get at all excited by one drug user with one baggie. Nor can I imagine the drug addict whose addiction will be broken by having one baggie confiscated.