OPINION: It seems crazy that in New Zealand we feel it necessary to march in the streets to affirm one of the most self-evident truths of all – the importance of science.

The marches were a response to the Trumpian denial of science and the danger was that in New Zealand the impression would be given that our government feels the same way.

There were signs protesting government policies, and that was always going to happen (it is election year, after all), but the marches achieved their aim, leaving everyone feeling buoyant and optimistic about science.

I hope it is the start of a trend. Any group feeling downcast or under-appreciated should feel free to organise their own march. Dairy farmers, police, hospital staff – they all must be finding life stressful and would feel uplifted. Not telephone salespeople, politicians or journalists, though. Anyone too far down the popularity list could run the danger of courting widespread derision.

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Jon Morgan: We're losing our judgment over cannabis

On the same morning, a small protest against the government's disinterest in cannabis law reform was also held. This started out as a call for the legalisation of medicinal cannabis but inevitably broadened to include social cannabis.

Despite calling down a firestorm of ridicule and hatred on myself for speaking out against cannabis legalisation recently, I am returning to the subject. My views have modified slightly since then.

I always was in favour of medicinal cannabis and a recent science review has reinforced that. The new bit is that I might - might - also be persuaded of the relative harmlessness of allowing a plant or two to be grown in one's home garden for personal use. But I remain adamantly opposed to commercialisation of cannabis in the same way as alcohol and cigarettes.

It is ironic that the protest organisers chose the same day as a science march because if it is one thing the most fervent cannabis smoker refuses to acknowledge it is science.

The recent science review of the subject by the US National Academies of Sciences, Engineering, and Medicine encompasses 10,000 studies and concludes there are good and bad points to cannabis.

The good reinforces the place of medicinal cannabis, saying it is promising for chronic pain, multiple sclerosis, and cancer patients.

But the bad confirms my view that, taken socially, cannabis is harmful. According to the science, it poses a significant risk for respiratory problems if smoked, schizophrenia and psychosis, car crashes, lagging social achievement in life, and perhaps pregnancy-related problems.

Respiratory problems shouldn't be a surprise; psychotic episodes are more likely in susceptible people, also not a surprise; that smoking mothers can give birth to smaller babies is also well-known; and the work-shy stoner is a caricature but one based on evidence.

Car crashes? It shouldn't be a surprise that our driving ability is impaired by cannabis. And this is where I have concerns about allowing home-grown cannabis. Those smoking it – and it is very much a social drug – would have to be vigilant about their friends' driving. How easy would that be? Reality becomes distorted pretty quickly.

I re-emphasise the point – we can have one drink and still drive safely, but not one joint. That's the difference with alcohol. Do we want another harmful social drug? Not I.

Jon Morgan is editor of NZ Farmer