Foreigners who use marijuana legally in Canada can be banned from entering the U.S. for smoking a single joint, a top border official said Tuesday, on the eve of Canada allowing the drug's recreational use.

Outlining broad discretion for border agents, U.S. Customs and Border Protection executive assistant commissioner Todd Owen said that smoking marijuana could result in being excluded from this country.

"Is it possible to be not let into the States by saying, 'Yes, I did smoke a joint', or 'I have smoked in the past?'" Owen was asked. "Yes," the senior border official replied. "Admission of illegal drug use are [sic] grounds to be found inadmissible into the United States. So yes, that's possible."

Owen, who was speaking on a conference call with journalists, emphasized that marijuana remains illegal under U.S. federal law — but also said discretion for agents could allow for expanded flexibility. "The officer always has wide discretion to ask about illegal drug use during the inspection process, and based on the answers, they also have wide discretion on determining admissibility," he explained.

"It's now legal in Canada, so a lot of it comes down to ... whether the officer believes they may engage in the same activity while in the United States," he said. "If somebody admits to smoking marijuana frequently in Canada, then that will play into the officer's admissibility decision on whether they think on this specific trip they are also likely to engage in smoking marijuana in the United States as well."

Past marijuana use and criminal convictions long have been grounds for being denied entry to the U.S. — infuriating reform advocates who point out that since 2012 nine states, the nation's capital, and one territory have legalized recreational possession, even if federal law still bans the drug.

"It’s totally up to the discretion of agents," Washington state defense attorney Douglas Hiatt, a marijuana reform advocate, told the Washington Examiner last week. "Those border agents have damn near plenary power, and it’s abused quite frequently."

Michael Collins, interim national affairs director of the pro-legalization Drug Policy Alliance, said that he could imagine controversy if someone is excluded from the U.S. after telling border agents they are visiting to attend a conference or to give a marijuana-related speech.

“CBP is going to back themselves into a corner and make a fool of themselves,” Collins said.

In the run-up to Canadian store openings, CBP created confusion last month with a vague statement warning Canadians that “working in or facilitating the proliferation of the legal marijuana industry in U.S. states where it is deemed legal or Canada may affect admissibility to the U.S." The agency clarified last week that cannabis workers might be allowed to enter the U.S. "for reasons unrelated to the marijuana industry."

Owen and other officials on the Tuesday call emphasized stability in U.S. policy, but offered little clarity on what guidance is in place for agents.

"Our protocols are not changing tomorrow, we would not expect [to be] routinely asking people that question," Owen said. "Not everybody coming in will be asked that question." Owen warned that visitors should not lie to border agents — a potential federal crime.

"You definitely don't want to be less than truthful when you're being interviewed by a CBP officer," he said. "The best course of action is always to be truthful with the officers when you are presenting yourself."

Owen also warned against mailing the drug into the U.S. or attempting to transport it over the border. Although adult recreational possession is legal in several neighboring states — Washington, Vermont, and Maine — crossing borders with pot is not allowed.

More than 100 retail recreational marijuana stores are expected to open in Canada on Wednesday. The legal purchase age is 18 or 19, depending on province. In states that regulate recreational sales, the minimum age is 21.