Marijuana has been legal in Canada for a month already, but immigration lawyers and cannabis executives say when it comes to getting into the United States, the worst may be yet to come.

As Canadians get used to the fact that cannabis is no longer against the law in their country, some experts fear they will forget the perils that past and present marijuana use still poses for those seeking to cross the Canada-U.S. border.

Henry Chang, a Toronto-based immigration lawyer, says he's bracing for a spike in cases of people who end up being banned outright from entering the U.S. for owning up to using pot.

U.S. Customs and Border Protection officials have made it clear that anyone who admits to using marijuana prior to Oct. 17, the day it became legal in Canada, could be banned from entering the country.

And Chang says U.S. law can still keep out anyone deemed to be a drug abuser or addict, or who is diagnosed with a mental disorder with a history of related harmful behaviour — including alcoholism or marijuana use.

Investors and employees in the cannabis industry, too, are on shaky ground — one U.S. executive says the risk of being banned for life from crossing the border has become a major preoccupation for his Canadian colleagues.

I think we're going to start seeing more people getting banned … because they think they have nothing to hide and they blurt out that they smoked marijuana when they were 18. - Henry Chang, Toronto immigration lawyer

"The bigger issue is people thinking the slate has been wiped clean," Chang said in an interview.

"I think we're going to start seeing more people getting banned, not because of them smoking marijuana after Oct. 17, but just because they think they have nothing to hide and they blurt out that they smoked marijuana when they were 18. That's going to happen, because people just don't understand that it's still barred."

U.S. border authorities initially warned that any Canadian who gave off a whiff of pot involvement — from using the drug to working or investing in the industry — risked being banned or denied entry. They later softened that stance, saying industry workers would generally be deemed admissible so long as they were travelling for reasons unrelated to their work.

Troubles attending cannabis conference

Some Canadians travelling to MJBizCon, a major cannabis industry conference last week in Las Vegas, faced additional scrutiny at various border screening points.

U.S. border staff at the airport in Toronto steered several attendees through a secondary screening process, said Rod Elliot, of consulting and communications firm Global Public Affairs .

Elliot said he was one of roughly 25 people who were selected for additional screening — several of whom missed their flights, including him. He travelled to Las Vegas the next morning without incident.

A sign near Gretna, Man., warns travellers not to cross into the U.S. with cannabis. (Rémi Authier/CBC)

"There has been challenges for people going across the border," said Elliot, who said his company advises various clients in the cannabis industry. "It's pretty clear that the border officials are targeting people attending this conference."

Len Saunders, a Canadian immigration lawyer based in Blaine, Wash., told a similar story about a group of would-be attendees and investors who were travelling through the airport in Vancouver. One of them, however, was part of a tour of a Vegas-based production facility in which he'd recently become an investor, arranged by his financial adviser.

That investor was unaware of the risks he faced crossing the border, and must now contend with having been banned from entering the U.S., Saunders said.

Proposed bill could make for safer passage

In the U.S., 10 states plus Washington, D.C., have legalized recreational cannabis use. But many in the industry — still hamstrung by the fact marijuana remains a controlled substance at the federal level, making it all but impossible to effectively obtain financing and other essential services — are hanging their hopes on bipartisan legislation aimed at ensuring states are protected from U.S. government interference.

The bill, unveiled earlier this year by Republican Senator Cory Gardner of Colorado and Democratic counterpart Elizabeth Warren of Massachusetts, is beginning to get traction in the wake of a strong Democratic showing in the midterms, said Derek Peterson, chief executive of California-based pot producer Terra Tech Corp.

"That, I think, will be the answer to legalization, and we will start to see some attention paid to that" in the new year, Peterson said.

"I think [Republicans] now understand that most of the constituents in the U.S. want to have some sort of federal regulation, a tax-and-regulate program, rather than just kind of leaving it with the black market, for all practical purposes."

'Don't dress like a hippie'

In the meantime, Chang has some common-sense advice for anyone who might have reason to be anxious entering the U.S.

"Don't dress like a hippie, don't smell like marijuana, because then the questions get asked," he said.

"If you are asked the question, your only option is to refuse to answer, say it's irrelevant. You refuse to answer — you'll get into trouble, they'll detain you, you'll get sent back to Canada, but at least you don't have anything on the record saying you engaged in controlled-substance use."