Cannabis would not have to be an exception to this practice, however, the fact is that it has many more uses and is far less dangerous than most other readily available palliatives. Then there is the question of hemp. The suppression of hemp by the American prohibitionist movement may have been a means to an end or an end in itself, but it is backfiring on the prohibitionists. Hemp can be grown with very low levels of psychoactive ingredients, but its wide spread cultivation for non-drug purposes may make the suppression of hemp grown for drug use even more problematic. This is not so much the result of the plants looking the same, but rather because it is difficult to demonise an agricultural crop over a long period of time.
Facts get even more in the way when they are growing in farmers' fields.
Finally, there is the success of the Dutch model of selling cannabis in coffee shops. This has worked very well. The reality of this success takes this subject out of the realm of speculation. We need not ask ourselves what might happen if we legalise cannabis. Failing to follow this success is not merely stupid, but morally wrong.