An admission of any drug use can cause an immigrant visa to be denied. This is because the immigration law states that an individual is inadmissible to the United States if he or she admits to the elements of a drug crime. In other words, a conviction is not necessary. Just admitting to prior drug use is sufficient.
Fortunately, USCIS does not routinely ask specifically about prior drug use at interviews for permanent residence or naturalization. However, consular officers abroad frequently do so at immigrant visa interviews, and there have been recent reports of Customs and Border Protection (CBP) officers asking such questions of noncitizens entering the United States. (Moreover, a person entering the U.S. at a port of entry might be asked by a CBP officer to share the contents of a cell phone or laptop, and must comply; he or she has no right to consult with an attorney.)
This issue, while in the immigration laws for decades, is becoming more problematic due to discrepancy between federal and state law with respect to marijuana use. A person who has previously spent time in a state where marijuana use is legal – Colorado, for example – might think that admitting to having used marijuana in that state is not going to cause a problem because it is legal there. In fact, if the CBP officer knows that the applicant was previously in a state known for legalized marijuana, the officer may be more likely to ask about marijuana use and to use any admission of such use to disqualify that person from entry. Also, even a noncriminal summons – payment of a fine for marijuana use – can result in exclusion or deportation from the United States.