Visitors to the U.S. risk being deported or denied entry if they smoke cannabis in the 11 states that have legalized recreational marijuana.

An immigration lawyer in London said that her firm has seen an increase in cases where vacationers and green card holders are being expelled for smoking weed. Charlotte Slocombe, a senior partner at Fragomen, said she has spoken with that other firms specializing in immigration to the U.S. and they have also noticed an uptick in deportations, people being denied entry and people being banned for life as a result of using marijuana in states like California and Colorado.

Marijuana is legal for medical purposes in 33 out of 50 states, while 11 have legalized recreational use. However, it remains illegal at a federal level and U.S. Customs and Border Protection Officers apparently keep having to inform visitors that the federal government still prohibits it.

Slocombe used the example of police raiding a party in a state like California or Nevada, where adult-use cannabis is legal. She said the police could not arrest Americans for smoking weed, but any foreigners could be arrested, deported and banned from returning to the U.S.

She warned visitors against going into an adult-use dispensary and handing over identification to make a purchase. Slocombe said that could trigger an alert for the authorities, who would know that the visitor has been in breach of federal law by using cannabis.

Immigration is covered by federal law, so the visitor could be kicked out of the country. “This is how people get caught out, even though they think they are doing something which is now legal in that state,” she said.

She said green card holders are similarly at risk, because they are subject to federal immigration laws. She also highlighted foreign investors in the U.S. cannabis industry having their investments deemed illegal under federal law, while a farmer struggled to renew his visa for selling a portion of his land to a licensed cannabis company.

Earlier this year, U.S. Citizenship and Immigration Services issued a bulletin stating that marijuana use is a disqualifying factor in citizenship applications, regardless of whether it is legal at a state level.