NEW YORK (NYTIMES) - He had no history of heart problems. He walked his dog regularly and worked a physically demanding job as a construction worker, according to his doctors.

Then, in January 2019, he collapsed at a McDonald's and died.

The likely culprit? Black licorice, according to the doctors who treated him and who this week published their findings about the unusual case in The New England Journal of Medicine.

The report said the man, an unidentified 54-year-old from Massachusetts, had consumed one to two large bags of black licorice a day for three weeks.

That habit caused his potassium levels to drop precipitously, prompting a cardiac arrest, according to the study. He never regained consciousness after his collapse, and died about 24 hours after he arrived at Massachusetts General Hospital.

"We almost didn't believe it when we figured it out," said Dr Jacqueline Henson, who treated the man while she was a resident at the hospital. "We were all shocked and surprised."

Aspiring doctors are taught in medical school that black licorice contains glycyrrhizic acid, a plant extract that is often used as a sweetener in candies and other foods and can lead to dangerously low potassium levels if it is consumed in high enough doses.

But it is rare to see a case of someone dying as a result of ingesting too much of the candy, Dr Henson said.

The man in Massachusetts had a poor diet and smoked a pack of cigarettes a day, according to his friends and family, his doctors said. But it was a switch from red to black licorice three weeks before his death that doctors said proved fatal.

Dr Henson said she interviewed the man's friends and family members, and doctors ran multiple laboratory tests that confirmed the man's potassium levels were well below normal.

They studied his medical history, which included heroin use, though he had not used opiates for three years.

There was no family history of cardiac disease or other conditions that would have led to low potassium levels, said Dr Henson, who is now a fellow at Duke University School of Medicine in Durham, North Carolina.

"We had no other clear cause for why his potassium levels were so low," she said.

The case "raises a public health issue that consuming large amounts of licorice can be hazardous to your health," said Dr Neel Butala, one of the authors of the study and a fellow at the interventional cardiology unit at Massachusetts General Hospital.

Dr Butala said consumers needed to be informed by candy and other food manufacturers about the levels of glycyrrhizic acid in their products. He also reported the case to the Food and Drug Administration (FDA).

In a statement, the FDA declined to comment specifically on the report, citing its policy not to discuss individual cases.

"The FDA is committed to protecting public health and ensuring the safety of our nation's food supply," the agency said in an email. "We are aware that the naturally occurring compound found in black licorice can have adverse health effects."

The FDA warns people who are 40 or older that eating two ounces of black licorice a day for at least two weeks can cause "heart rhythm or arrhythmia."

The agency warns consumers on its website about the dangers of an overdose from black licorice. The compound glycyrrhizin, which is derived from licorice root, can cause potassium levels in the body to fall and lead to abnormal heart rhythms, high blood pressure, edema, lethargy and congestive heart failure, according to the agency's warning.

Dr Henson said people who like to eat the occasional piece of licorice should not be alarmed by the case in Massachusetts. Black licorice is not a poison, she said.

"It's fine taken in sort of small amounts, infrequently," Dr Henson said. "But when taken on a regular basis, it can lead to these issues."