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my mate wont listen to the advice his friends are givin him an its startin 2 scare us all
As skive said it depends what he's taking it and how, if he's a desperate heroin junkie and lives each day by searching for the next hit then i'd start to worry about him but if he's only a weekend user out taking a few pills or some coke then that's what he wants to do and you probably don't have call for much worry.
If they're not ready to stop, there's pretty much nothing you can do, except let them know that you'll support them when they are ready.