banned

Canadians may be surprised to learn that United States citizens who have been convicted of (or who have committed) a single instance of Driving under the Influence ("DUI") will actually be barred from Canada. Some U.S. citizens may believe that this is unfair also, especially since Canadians who have DUI convictions are generally not barred from the United States.
Matthew Harvey, a British Columbia resident, was permanently banned from the United States because he admitted to a U.S. Customs and Border Protection (USCBP) officer in 2014 that he had previously smoked marijuana. But why does this occur and what can be done if it happens to you?