This is a british study.
http://www.rcpsych.ac.uk/healthadvice/problemsdisorders/cann...
> A young person found to be in possession of cannabis will be:
> Arrested
> Taken to a police station
> Given a reprimand, final warning or charge, depending on the offence.
> After one reprimand, a further offence will lead to a final warning or charge.
> After a final warning:
> The young person must be referred to a Youth Offending Team to arrange a rehabilitation programme.
> A further offence will lead to a criminal charge.
That's pretty far from being imprisoned for a single puff.