Revealed: Patrick Gower learns what cannabis does to your IQ

After sampling cannabis himself, Newshub's national correspondent Patrick Gower is now trying to find out about the negative effects.

The internationally-renowned Dunedin Study has tracked 1000 New Zealanders for the past 46 years and has the world's best data on the long-term effects of cannabis usage.

"If you start early - and by early I mean in the adolescent years, and persist in your use, then the likelihood of developing a number of difficulties all increase," says Professor Richie Poulton, the director of the Dunedin Study.

"During that period, the brain's undergoing a lot of changes, so it's particularly sensitive to the effects of - in this case - THC.

"We've shown in a much-cited piece of work that if you start using during adolescence and you continue using on a regular up to the age of 38, that if you're in the highest use group your IQ points will drop by eight."

Gower inside a grow-patch.
Gower inside a grow-patch. Photo credit: Newshub

Prof Poulton says the chance of developing psychosis also increases.

"If you start using cannabis during mid-adolescence, and you have a certain genetic makeup, your risk of becoming psychotic by the time you're in your mid-twenties is increased 11-fold," he tells Gower.

Professor Joseph Boden is the director of the Christchurch Health and Development Study, which has been tracking 1200 Kiwis for 42 years. He has more bad news.

"There are several important issues arising. So the first is what we call an 'a motivational syndrome'," he says.

"Essentially what it means is if you start using cannabis more heavily at a younger age, you're less likely to achieve your educational qualifications.

"You're more likely to experience longer periods of unemployment and you're more likely to become welfare dependent."

Professor Joe Boden.
Professor Joe Boden. Photo credit: University of Otago

And some people may become dependent on marijuana itself.

"There is a cannabis dependent syndrome. It includes withdrawal symptoms with headaches and related fatigue," Prof Boden says.

"About 15 percent of the people who use cannabis become dependent at some point in their lives."

So after trying cannabis twice, how much is Gower at risk?

"If you use cannabis occasionally for recreational purposes there are very few negative effects," Prof Poulton says.

"To be completely honest, and it sounds like what your grandma says, use everything in moderation, and our take-home message for young ones is stay clear of substances, cannabis, until you're around the age of 18."

You can watch the full documentary on ThreeNow.