In the final instalment of Daybreak's "Montreal 420" series, a retired Montreal police officer discusses how legalization could change the city's illicit drug trade.

Philippe Paul spent 28 years with the Montreal Police force, most of that working on the drug and anti-gang squad.

Paul is also an expert witness in narcotics trafficking cases and author of the book Coupable d'etre policier.

Our series Montreal 4-Twenty continues, with a look at the black market in marijuana with retired Montreal police officer Philippe Paul. 9:45

Here are key excerpts of his interview with Daybreak host Mike Finnerty.

How big is the illicit sale of marijuana ?

Marijuana is a very "large part of organized crime," says former police officer Philippe Paul.

It's quite significant. Quebec-grown marijuana is in demand, in Canada and elsewhere in the world.

It's a very large part of organized crime. At least 50 per cent of the narcotics sold here in Quebec is marijuana.

What might happen if we legalize marijuana in two years time?

Paul believes legalization will do little to reduce criminality in Montreal. (RCMP)

It will have to be controlled because the THC level in Quebec-grown marijuana is very high and it can become addictive. It has to be grown in a proper fashion. You can't just let anybody anywhere grow and sell it.

It will be difficult because it will always be readily available and grown by anybody.

What effect might legalization have on dealers?

Kelowna tops the list of 34 Canadian cities for the highest per-capita rate of marijuana possession charges in 2014. St. John's had the lowest rate. (CBC)

That's the good part. If it's controlled, and grown by the government, that's one less valve open for organized crime. So that's less money for them, so they have to turn to other illicit drugs.

What do you think of legalizing marijuana?

Marijuana will force organized crime to find other sources of income, Paul says. (Deux-Montagnes regional police)

Obviously, as a former police officer, I'm not for that. I was fighting crime and drug-trafficking all my career and seizing illicit drugs including marijuana.

But, as I said, the good part of it is that if that if the drug is legalized, at least that's one thing that you're taking out of the hands of organized crime and other dealers in and around Montreal and elsewhere.

What are some of the challenges of policing marijuana use?

Controlling marijuana will be hard to do given how easy it is to grow, Paul says. (RCMP)

I've intercepted people driving under the influence of narcotics, so obviously it's not like alcohol. Alcohol you can smell the breath, you can see it in the eyes. With marijuana, if you really want to prove it in court, you need a blood sample and you need a warrant to get that. It becomes very long and difficult.

Do you think legalization will reduce criminality in Montreal?

Paul is skeptical legalization would reduce criminality in Montreal. (Jeff Chiu/Associated Press)

Not at all. Not at all. I don't think it will change a thing.

The interview has been edited for clarity.