A CANADIAN woman was denied entry to the United States last month because she had been hospitalized for depression in 2012. Ellen Richardson could not visit, she was told, unless she obtained “medical clearance” from one of three Toronto doctors approved by the Department of Homeland Security. Endorsement by her own psychiatrist, which she could presumably have obtained more efficiently, “would not suffice.” She had been en route to New York, where she had intended to board a cruise to the Caribbean.

“I was so aghast,” Ms. Richardson told a Toronto Star reporter. “I don’t understand this. What is the problem?’ I was so looking forward to getting away. I’d even brought a little string of Christmas lights I was going to string up in the cabin.”

The border agent told her he was acting in accordance with the United States Immigration and Nationality Act, Section 212, which allows patrols to block people from visiting the United States if they have a physical or mental disorder that threatens anyone’s “property, safety or welfare.” The Star reported that the agent produced a signed document stating that Ms. Richardson would need a medical evaluation because of her “mental illness episode.” A spokeswoman for United States Customs and Border Protection told the Star that the agency was prohibited from discussing specific cases because of privacy laws.

This is not the first time such measures have been reported. In 2011, Lois Kamenitz, a Canadian and a former teacher, was barred from entering the United States because she had once attempted suicide. Ryan Fritsch, former co-chairman of the Ontario Mental Health Police Record Check Coalition, told the Star that he had heard of eight similar cases that year. After the incident, he wrote to me: “My sense is that there are a great many people being turned away. I’ve also heard of executive-level reps from various Canadian and provincial mental health advocacy and awareness organizations being turned away at the border on their way to conferences and other official functions and appearances,” presumably because of their own medical histories.