The advent of party pills, Ecstasy and other synthetic psychoactive substances took the law by surprise a decade or so ago.

They could be sold as "legal" drugs because they were unknown to the Misuse of Drugs Act 1975, which was written when recreational drugs of concern were primarily cannabis and LSD.

In 2007, the previous Government asked the Law Commission to look at the new substances as part of a review of the act. This week, without much fanfare, the commission produced its conclusions.

It proposes setting up an authority in the Ministry of Health for new psychoactive substances, and that manufacturers or importers should need approval of a substance before it can be distributed.

That would deprive the drugs of the presumption of innocence they now enjoy. As the law stands they can be sold unless and until they are found to cause harm.

It takes time for harm to be established, but that is not the main reason a mood-altering pill should be banned until proven safe.

Party-pill manufacturers have shown themselves capable of substituting a pill's chemical composition with another as soon as the first is found harmful, and the substitute might be worse, as health authorities discovered when a product claiming to be a non-neurotoxic substitute for Ecstasy went on sale in 2005. It took several months to discover that the new ingredient was an analogue of an already prohibited Class C drug.

But if party pills are not to be allowed on to the market until a health official is satisfied they are safe, party pills of any potency are probably not going to be seen again. This is a risk-averse age. Safety is a hard test to satisfy, particularly where health is concerned.

If society's most common recreational drug, alcohol, was applying for approval from a health and safety regulator today, would it get it? Its sale is subject to age restrictions, one of which Parliament is about to raise from 18 to 20 at the urging of public health campaigners and the Law Commission.

But the principle remains that the law should not, without very good reason, restrict what people consume voluntarily at a risk to nobody but themselves.

Drugs, as the commission's report notes, have been used since antiquity. It was not until the late 19th century that Governments started trying to prohibit some of them.

Opium was legal in New Zealand until 1901.

Referendums on the prohibition of alcohol started in 1902 and one nearly succeeded in 1919.

Restrictions on alcohol sales began to be eased in the 1960s and gathered pace in the last quarter of the century, but other drugs did not enjoy the same leniency.

Tobacco came under increasing control after its harm was confirmed. Cannabis, still a widely used illicit substance in New Zealand, is no closer to legalisation than it has ever been, though the commission suggests a relaxation of penalties for possession for personal use.

New Zealand's national drug policy states that restrictions are justified if the drug user can cause harm to others, or lacks the necessary information, maturity or ability to assess the risks.

If the Government decides party drugs must pass that test before they are put on sale, their days are probably numbered.