Justin Bieber -- I'm Not a Druggie ... I'm Just 19

Justin Bieber is laughing at everyone who is worried about his drug abuse ... because he thinks what he's doing is simply the mark of a 19-year-old ... and besides, it's fun.

People close to Justin tell us ... he's laughing at the stories TMZ has posted about his drug use ... in particular, what has become an out-of-control sizzurp problem. Justin's feeling is that he's young with lots of time on his hands -- as he puts it, "I'm on break." There are no concerts or other obligations on his immediate schedule.

As we reported ... Justin's people have been urging him to get help for what has become a bad drug problem, but he has shut them down.

In fact ... over the weekend, Bieber posted a video on Instagram on an ice rink with the hashtag "worried bout nothing"

Check out the video.