U.S. Customs and Border Protection (CBP) enforces the laws of the United States and U.S. laws will not change following Canada’s legalization of marijuana. Requirements for international travelers wishing to enter the United States are governed by and conducted in accordance with U.S. Federal Law, which supersedes state laws.
Although medical and recreational marijuana may be legal in some U.S. States and Canada, the sale, possession, production and distribution of marijuana or the facilitation of the aforementioned remain illegal under U.S. Federal Law.
Consequently, crossing the border or arriving at a U.S. port of entry in violation of this law may result in denied admission, seizure, fines, and apprehension.
Generally, any arriving alien who is determined to be a drug abuser or addict, or who is convicted of, admits having committed, or admits committing, acts which constitute the essential elements of a violation of (or an attempt or conspiracy to violate) any law or regulation of a State, the United States, or a foreign country relating to a controlled substance, is inadmissible to the United States.
A Canadian citizen working in or facilitating the proliferation of the legal marijuana industry in Canada, coming to the U.S. for reasons unrelated to the marijuana industry will generally be admissible to the U.S.
HOWEVER, if a traveler is found to be coming to the U.S. for reason related to the marijuana industry, they may be deemed inadmissible.