A man says he was turned away from the U.S. because a police officer caught him smoking marijuana when he was 17, more than two decades ago.

After years of travelling south without issue, the man says he was stopped at Pearson International Airport before boarding a plane for a business meeting earlier this month.

The man, who wished to remain anonymous to protect his privacy, says he was pulled aside by Homeland Security and told he was convicted of narcotics possession.

"I said, 'Pardon me? Narcotics possession? What are you talking about?'"

He told CTV Toronto's Pat Foran that 24 years earlier, he was caught smoking a joint of marijuana in an Oakville, Ont. park. He and a group of friends were given a fine, but didn't go to jail or face any other punishments.

"My parents were involved, they were aware of the whole thing. I was a teenager," he said. Since the conviction, he's been travelling to the U.S. for work for years. No one had ever stopped him before.

He told CTV Toronto he's devastated, and his business will suffer if he can't get back into the U.S.

Though the man was shocked to be prevented from travelling, a Canadian immigration lawyer said he wasn't surprised. U.S. officials have been requesting more criminal information from Canadian authorities in the last few years, leading to more people being denied entry.

"The protocols on the sharing of information between Canada and the United States since 9-11 have dramatically increased," immigration lawyer Joel Guberman told CTV Toronto.

"All kinds of information that's in Canada's files is now in the U.S. files, and every day they go deeper and further back," he said.

A conviction for theft, fraud or assault is also grounds for denial of entry.

To be allowed back into the U.S., Canadians can apply for a waiver, but the process can take a year and must be applied for annually. The U.S. does not recognize Canadian pardons, Guberman said.

"The best thing one can do is apply for a waiver early and unfortunately often."

With files from CTV Toronto's Pat Foran