SARAH FERGUSON, PRESENTER: Earlier this year, Colorado became the first state in the US to allow fully-legalised marijuana to be sold over the counter. It's the culmination of a massive shift in public attitudes and for the first time polls show a majority of Americans are now in favour of legalisation. So what happens in Colorado could well set the model for other states to follow. Three months in, our North America correspondent Ben Knights went to Colorado to see how it's going.

BEN KNIGHT, REPORTER: This is how easy it is to buy marijuana in Colorado.

I'm not the one who's shopping. This guy is.

His name's Jake Browne and he's a marijuana critic with the Denver Post newspaper.

And this is what he's here for: to review the product of one of the state's newest marijuana outlets.

It was three months ago that Colorado's medical marijuana stores got the green light to start selling to any adult who walked in.

My 21-year-old self can't quite believe this is happening. In fact my 43-year-old self can't quite believe this is happening.

JAKE BROWNE, MARIJUANA CRITIC, DENVER POST: It's fun to be able to do this without having to pretend that I've got a back problem or I can't sleep. It's just we're here as adults checking out some recreational marijuana.

BEN KNIGHT: And already, the legal industry here is huge. In the first month, $14 million worth of recreational marijuana was sold in Colorado just like this.

TONI SAVAGE FOX, 3D CANNABIS CENTER: I have my cultivation facility on site here. We have the world's only garden viewing corridor where people can actually come and see the cannabis being cultivated first-hand. Would you like to go inside?

BEN KNIGHT: I'd love to.

TONI SAVAGE FOX: Alright.

BEN KNIGHT: Toni Fox has made more money in the last three months than she did in three years selling medical marijuana.

TONI SAVAGE FOX: There are tour buses and charter buses from out of state that come in that are full. I have limousines that pull up every day. There's at least a dozen tour companies that I personally know of and many of them are booked out months in advance.

BEN KNIGHT: Colorado is reaping the benefits of legalising marijuana first. There were predictions that this year the state will take in somewhere between $60 and $100 million in marijuana taxes. Four other American states are expected to follow soon. But Colorado is the test lab. The rest of the country is watching what happens here very closely.

People try to compare what's happening here in Colorado to what's been happening in Amsterdam for years, but that's not true because in Amsterdam, it's only tolerated. Here, it's 100 per cent legal. If you're 21 years old, you can go and get it and you can use it. Now there are some restrictions of course. You can't smoke in public, you have to do it a home, you certainly can't drive under the influence and if you turn up to work stoned, you're going to be in just as much trouble as you were before.

So, what does a big modern city with legal marijuana look like? Well, pretty much the same as it did before. Quite simply, this isn't an earth-shattering change in a state where, for more than a decade, medical marijuana's been easily available to pretty much any adult who wanted it.

BOB ESCHINO, INCREDIBLE EDIBLES: The sky didn't fall. Right? There wasn't 1,000 people in the ER. There wasn't accidents all over the streets.

BEN KNIGHT: It was back in 1996 when the first ever medical marijuana co-op opened in San Francisco. Back then, only around a quarter of Americans supported legalising the drug. Today, more than 20 US states allow medical marijuana and public support for legalisation is closer to 60 per cent.

Yet, under federal law, this is still an illegal drug. And the only reason a business like this can operate, making marijuana-laced chocolates, is because of what amounts to a promise from President Barack Obama to those states that federal agents won't enforce the law there, and that's a problem when it comes to banks, who don't like that kind of uncertainty.

BOB ESCHINO: We're not allowed to have credit cards, bank accounts, access to capital. Just a line of credit would be life-saving at this point. We're growing at such a rate, it's hard to keep up.

BEN KNIGHT: So every single marijuana business can only operate with cash.

What happens to the cash? It's a cash-only business.

BOB ESCHINO: (Laughs) I can't talk about that. (Laughs)

BEN KNIGHT: This industry's growing so fast, that this month, Denver held a special marijuana job fair.

MAN: When I seen this on Facebook, I jumped in my car and drove for the last 13 hours to get here.

BEN KNIGHT: Other states look on. They see the jobs, the tourism and the tax revenue and they take note. And that is what worries opponents of legalisation.

KEVIN SABET, ANTI-MARIJUANA CAMPAIGNER: We're creating the next big tobacco of our time.

BEN KNIGHT: Dr Kevin Sabet is the highest profile anti-marijuana campaigner in the US.

KEVIN SABET: We are now creating an industry whose only business is to increase addiction. And now we see millions of people in the US in addiction treatment for marijuana. We see young kids now getting the message that using marijuana is OK.

ANDREW MURPHY, ELDER, BIBLE WAY CHURCH: I don't understand how people can be so blind and think that there's nothing wrong with it when you come to a black community and you see what's going on.

BEN KNIGHT: Like the Shaw neighbourhood in Washington, DC.

The nation's capital leads the country in marijuana arrests and more than 90 per cent of those arrested are black.

Andre Murphy started smoking marijuana in his teens, and like many of his friends, quickly moved on to heroin, crack and prison. He's been clean for 20 years. He's now a church elder, ministering to addicts on the street. He doesn't want to see more black kids going to jail, but he does not support legalising marijuana.

ANDREW MURPHY: You're playing with fire because we thought - we wanted to use it recreationally, but it didn't turn out - it didn't end that way. It ended up with us being addicted to something even worse than what we started out with.

BEN KNIGHT: But here in Colorado, legalisation is not the end of the process. Next comes normalisation. So, instead of just smoking on his couch, Jake Browne writes a review that will be published by a major city newspaper.

Bobby Eschino has a dream: to sell his products at baseball and football games.

BOB ESCHINO: We've kind of changed the face of the cannabis industry here and I think we want to spread that across the country.

KEVIN SABET: You almost had to legalise it in a few places for people to wake up and say, "Wait a minute, I thought I was voting for an adult to be able to smoke a joint and not be a criminal in his own home. I didn't realise I was voting for a pot shop near my kid's school."

SARAH FERGUSON: Ben Knight reporting.