OTTAWA -- The former U.S. ambassador to Canada warns the new marijuana law could effectively thicken the border as sniffer dogs trigger searches for marijuana that’s illegal in the United States, but legal in Canada.

Bruce Heyman, who served as U.S. ambassador in Ottawa under former president Barack Obama, says he looked into how legal marijuana could impact the border.

Heyman was envoy from April, 2014 until January, 2017, as the then-opposition Liberals promised to legalize marijuana and promised in the first year of their government to carry through with the pledge. Heyman says, when he spoke to American border officials at the time, they raised the issue of how sniffer dogs at the border would handle the smell of pot.

"The dogs are trained to have reactions to certain scents. Some of those scents start with marijuana. Others are something that are significantly more challenging for the border. But the dog doesn't tell you this is marijuana and this is an explosive," Heyman said.

"The dog reacts, and these border guards are going to have to appropriately do an investigation. That could slow the border down."

Heyman says the dogs are trained from a very young age and that training can't be undone. Changing how they react to marijuana would require getting new dogs, he said.

The additional searches trigged by the dogs, Heyman said, might slow down border crossings.

"In the specific example that you raised, it has that possibility," he said.

Different language on trade

A thickening, or slower-moving, border would impact the flow of goods and people between the U.S. and Canada, one of a range of trade concerns facing the two countries.

U.S. President Donald Trump has threatened to tear up NAFTA, last month getting as close as being a few days from giving official notice that America would tear up the trilateral agreement. The Americans have also imposed tariffs on Canadian softwood, a process that got underway when Obama was still in office.

Trump scaled back his comments during Trudeau's visit to Washington, telling reporters at a joint press conference that the two countries - plus Mexico - would be "tweaking" NAFTA.

But Heyman says that remark is an outlier.

"He's threatened all throughout the campaign that he was going to tear up NAFTA," the former ambassador said.

"That was a very clear and repetitive dialogue that he's had all through the campaign. It was only at one day, at one time, where he used the word tweak... So I think that was the exception, the tweak, rather than what was being consistently communicated."

Heyman, however, says the trade irritants that exist are small compared to the overall relationship between the two countries.

"I don't think the differences are any bigger now than they were before. I think the language being used is different now," he said.

Importance of cross-border trade

Meanwhile, when asked about the issue of a thickening border due to the effects of cannabis, the Public Safety Minister’s office said the federal government continues to work on the details of legalizing the substance.

But in a statement to CTV News, Minister Ralph Goodale's spokesman Scott Bardsley said the office is “keeping in close touch with the United States about the legalization of cannabis.”

Bardsley stressed the importance of a strong cross-border relationship between the U.S. and Canada in order to facilitate trade and travel while securing the countries from shared threats.

“400,000 people and $2.4 billion in trade cross our shared border every day. Both countries recognize the importance of an efficient and secure border for our shared prosperity.”