NZ '4000 years behind' on medicinal marijuana - expert

A visiting expert on medicinal cannabis says New Zealand is 4000 years behind the times when it comes to adopting the "very effective" medicine.

Dr David Bearman, author of Drugs are NOT the Devil's Tools, has spent 40 years working in and researching substance abuse treatment and prevention.

He's here to talk to doctors, medical students and politicians, and encourage them to treat cannabis as the medicine it truly is.

"Except for the period from 1942 until 1996, cannabis has been a medicine for the last 4000 years - a very effective medicine and a very safe medicine," he told The AM Show on Thursday morning.

California was the first place in the world to start letting its citizens use marijuana for medicinal purposes. In New Zealand, usage is still heavily restricted, with individual patients requiring approval from the Ministry of Health.

"I want to encourage the medical community here to recognise cannabis not only as a useful medication, but has very few side-effects," says Dr Bearman.

When he first saw the list of conditions supporters of medicinal marijuana can help with, Dr Bearman says he couldn't believe it.

"I thought no one compound, no one medicine can treat all of these conditions," including ADHD, post-traumatic stress disorder, nausea, appetite stimulation, migraine headaches, seizures and even cancer, "and yet it can."

That's because cannabis works by hooking into the body's endocannibinoid system, which affects "almost every organ" in the human body.

"We all have our marijuana-like substances that we make, and that goes back in evolution to trilobytes," he explains.

"It's been around for a couple of hundred million years, and it was here well-before the cannabis plant was here."

Humans used cannabis for both recreation and therapy for thousands of years before anti-drug movements swept the world in the early to mid-20th century. It was banned in the US in the late 1930s, and in New Zealand in 1965.

It was made a Schedule I controlled substance in the US in the early 1970s, pending the outcome of a review - which found cannabis probably shouldn't be illegal at all, but President Richard Nixon decided to leave it so.

Dr Bearman says this was a huge mistake.

"There's been a big lie that has been told, and it's really a shame. We have people whose cancer we know has been ameliorated by cannabis... there's more than enough basic science and anecdotal reports to justify double-blind studies."

Associate Health Minister Peter Dunne has previously backed calls for clinical trials in New Zealand, saying there was nothing in the law to stop it happening.