A year ago, when the Government reversed a brief legalisation of "party drugs", we heard the prediction that is always heard when law is used in this area. Namely, that prohibition never works, that dealing in the banned substance would simply go underground, suppliers would prosper on an unregulated, untaxed market and users would be afraid to seek professional help before their addiction landed them in hospital emergency rooms.

A year on, the usual prediction appears well astray. Psychiatrist Dr Paul Glue told our health reporter this month the clampdown on synthetic cannabis had virtually stopped the flow of mostly young male patients into mental care suffering toxic effects. "You might see one every two or three months," he said. "It seems there is a very small amount of synthetic cannabis now. The number of people presenting with some sort of toxicity due to it is incredibly rare."

If the trade has gone underground it is not booming there. It was reported last weekend that one of those who made millions from party pills and legal highs, musician Matt Bowden, has had his company, Stargate Operations, put in liquidation. He told 3News his business struggled since the Government's banned synthetic cannabis and could not afford to pay outstanding bills.

So much for the black market. So much for the claim that prohibition never works. It might be futile for a drug as pervasive as alcohol, and it is clearly less effective against plant cannabis, but the lesson of the ban on chemical versions is that liberal policy is not an enlightened response in all cases.

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The Government, it turns out, was right to take these drugs off the shelves. It was wrong a year earlier when it was persuaded to give some of them interim legality while it tried to develop safety tests for new psycho-active substances that would permit manufacturers to put them on sale. Some of these so-called "legal highs" had been openly on sale for years.

When it decided to regulate them in 2013, it not only gave interim legality to some of them, it let local councils limit the number and location of outlets. The result was a social and political disaster. Permitted outlets were mainly in less salubrious shopping centres and the trade became highly visible. Residents and other retailers were appalled; drug users hung around playgrounds and other public places.

Distraught mothers of youthful users phoned radio talk shows and went on television programmes, notably Campbell Live, to testify to the damage these drugs were doing. By then it was election year and the Government soon called off its tentative liberalisation.

Now the Government is being urged to legalise plant cannabis for medical purposes. Associate Health Minister Peter Dunne, who was responsible for the party pill experiment, is wary. He has called the advice of his officials "underwhelming" on the medical case for legalisation. Their synthetic cannabis experience has given policy makers new respect for the force of law.