A US anti-cannabis campaigner has warned New Zealand against legalising recreational cannabis after seeing the effects of the drug in his home state of Colorado.

Marijuana was legalised for recreational use in Colorado in 2012, meaning anyone 21 years or older can use, carry and grow the drug there.

Ben Cort joined The AM Show today to tell his story of addiction to marijuana and how legalisation in some US states has led to increases in addiction and mental health issues.

"I spent five years at the University of Colorado hospital when we legalised and we went from seeing paranoia associated with it every now and again to multiple times in a day."

He said legalisation brings with it forms of the drug that have much higher THC levels.

"People don't understand that we're not talking about a joint.

"People are smoking vapourisers that come in the form of functional pens that you can write and then hit... it's not weed, it's a concentrate. An 80 percent THC concentrate."

In February, Green Party MP and pro-cannabis campaigner Chloe Swarbrick joined The AM Show to debate the topic, saying people currently aren't put off by the prohibition of cannabis, and legalising it would mean the industry could be regulated.

"That illegal drug dealer is not going to check your ID; they are not going to know the chemical balance of THC, CBD... within that cannabis; they are not going to be paying their tax; and worse than that, that is going to provide profit to a criminal underground."

She said she was not on-board with the Colorado model of legalisation to which Mr Cort was referring, where advertising for cannabis products can be plastered anywhere and it can be "delivered door-to-door".

"Legalisation is not liberalisation. I think what you're imagining is we're all of a sudden going to have this free-for-all - everybody is [already able] to access it under the present model."

Mr Cort said since legalisation, his 16-year-old daughter gets offered the drugs by friends weekly, some even smoking it at school.

"It's pretty intense back home. There's no day when I don't watch someone consuming."

He said legalisation hasn't stopped people from using the drug dangerously.

"The driving under the influence, the working under the influence - it has changed my home."

Ms Swarbrick said the majority of New Zealanders know how to get cannabis despite its illegality, and legalisation would stop dodgy trades and inject money back into the community through taxation.

"We want to see a sensible model that minimises harm - we want to educate, we want to regulate to ensure that this isn't seen as advertised, cool or promoted to kids - and we want to ensure the tax revenue... is invested back into mental health services."

Mr Cort said New Zealand should learn from examples in the US when deciding whether to legalise cannabis.

"You need to understand that we are not talking about the plant, the drug that people consumed in years past. It has fundamentally changed and that genie can't go back in the bottle."

"We have changed from a plant with two-to-three percent THC in it, to something that is 90-plus percent THC, put into sodas, water, gummy bears, tea, coffee, it is not the same drug."

The New Zealand Government will be holding a referendum at the 2020 election, to garner Kiwis thoughts on making cannabis legal.

Newshub.