REGULATION of cannabis sales in Canada has not killed off the black market overnight. Half the cannabis purchased across the country continues to come from illegal dealers.

But a 50 per cent cut in the illicit trade in less than a year is difficult to ignore. Many campaigners argue that the war on drugs has already failed and that a much more effective approach is to treat the problem as a public health crisis rather than a criminal one.

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In this way, selling cannabis over-the-counter, legally, offers the state the opportunity to package it in the same way as cigarettes - with graphic health warnings and drab plain packaging - while at the same time using taxes to raise revenue for the Treasury that would previously have lined the pockets of criminal gangs.

Striking the right balance is complicated, however. In Canada, sales of the world's most popular legal drug - alcohol - are already restricted to specific licensed premises, unlike the UK where is has been widely available in shops and supermarkets for decades.

As a result, it was natural for Canada to restrict cannabis sales to a limited number of specialist licensed dispensaries. But the question is, how many should there be?

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Toronto, for example, has only three cannabis outlets for a city of nearly three million people. As a result, some residents are still inclined to turn to their old black market dealer, who will bring it to their door.

Of course, illegal dealers still want a slice of the action and will undercut the legal sellers by offering a cheaper product.

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The inevitable effect of taxation - particularly where levies are targeted on a sliding scale to hit high-potency strains hardest - is that it opens the door to illegal gangs preying on poorer cannabis users with the lure of cut-price weed.

Obviously minors already get round the law on alcohol and cigarettes by using fake ID or asking adults to buy it for them.

In Canada, minors in possession of cannabis will not be prosecuted - but adults caught giving it to them can face long prison sentences.

How often this actually happens is unclear though. Such penalties must be enforced to be a deterrent.