VANCOUVER -- A Colorado official has some sobering words of advice for Justin Trudeau if he fulfills his promise to legalize pot in Canada.

"It's going to be a lot harder to implement than you think. It's going to take a lot longer to do it. And it's going to cost more than you think," said Lewis Koski, director of the state's Marijuana Enforcement Division.

Colorado is one of four U.S. states to fully legalize recreational bud. Their challenges -- including how to regulate edibles like brownies and cookies and a rise in drug-impaired driving -- could be instructive for Canada's incoming prime minister.

Among the questions Trudeau's government could grapple with are whether to allow people to grow pot at home or buy it in stores, and how much sales tax to charge.

In Colorado, adults over 21 can grow up to six plants at home, while those who buy recreational weed in stores pay 25 per cent sales tax on top of the regular 2.9 per cent sales tax.

The state has collected $141 million in taxes since storefront sales began in January 2014. But a portion of the tax earmarked for school construction projects has fallen short of a $40-million goal.

There's also the matter of how to regulate edible products, which often take the form of sweet treats that appeal to children or are so potent that adults easily overindulge.

Two suicides and a murder committed by people who consumed edibles have caused alarm in Colorado. The state introduced new rules in February to require more explicit warnings on labels and offer companies incentives to produce lower-potency goods.

In Washington, nearly half of marijuana poisoning calls last year involved children. Packages on pot products can't use cartoon characters or bright colours, and must clearly mark each 10-milligram serving of THC, the chemical in pot that makes users feel high.

"It can't be especially appealing to children, which is admittedly a bit subjective. So each one of those products is actually submitted for review prior to going on the shelves," said Mikhail Carpenter of the Washington State Liquor and Cannabis Board.

Another area Canada will need to study is drug-impaired driving. While fatal crashes in Washington only increased slightly after legalization, the percentage of drivers involved in those crashes who tested positive for THC doubled -- to 12 per cent in 2014 from 6 per cent in 2010.

There is no approved breath or saliva test in the U.S. or Canada to determine if someone recently consumed marijuana. In Washington, a blood test is the best available method to measure THC levels. The state's maximum is five nanograms of THC per millilitre of blood.

Washington police must obtain a judge-approved search warrant before bringing a driver to hospital for a blood test -- a process that can take a couple hours. While marijuana remnants can stay in a person's system for days, active THC dissipates rapidly.

"That's why in a general traffic-stop situation, where say a person was smoking it as they were driving down the road, there's a time aspect where we want to try to get the test done as soon as possible," said Washington State Patrol Sgt. Brandon Villanti.

But questions remain about whether every person with active THC in their system is actually impaired by the drug. Lawyer John Conroy said medical patients who regularly use marijuana do not get high.

"Just measuring nanograms in your blood doesn't do it. That's what they do in Washington state and you're presumed to be impaired and it's irrebuttable."

Alaska and Oregon legalized pot last year and are in the midst of crafting new rules. Cynthia Franklin, director of Alaska's Alcoholic Beverage and Marijuana Control Boards, said lawmakers are under pressure to meet tight timelines and begin issuing licences in May.

"It's a wild ride, and we've been through a lot of loops and twirls and stomach-churning drops," she said.

Representatives from all four states stressed the importance of public engagement. Oregon recently adopted its temporary requirements for marijuana licensees after a robust community debate.

"We don't have a bunch of controversy around our rules because we've been transparent and open," said Rob Patridge, chair of the Oregon Liquor Control Commission.

Patridge, also a district attorney, estimated about 75 per cent of medical marijuana grown in the state is currently going into the black market or being exported.

"We're hoping that the regulatory environment that we've put in place will attract those who want to participate in a legal market," he said.

"However, Oregon is way over-producing for its current population that could partake in marijuana. So, unless (legalization) is done on a nationwide level, we are certainly going to continue to have significant black market problems."

And if Trudeau wants to learn from Oregon, Patridge said he would welcome a visit.

"Tell him our door is open to him. We're happy to share with our neighbours to the north."