The problem was discovered when abnormally low oxygen levels were measured in the man's blood while he was under anaesthesia for an operation, doctors wrote in BMJ Case Reports.
Blood tests showed alarmingly high levels of cyanide.
When he woke up, the man told doctors he had been taking a daily dose of two teaspoons of homemade apricot kernel extract for five years. He also took three daily tablets of a herbal fruit kernel supplement.
The man was taking more than 17 milligrammes of the extract every day, enough to raise blood cyanide to about 25 times acceptable levels, the team said.
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According to medical websites, 50 milligrammes -- about one to two teaspoons -- of the poison taken orally would be enough to kill a person weighing 160 pounds (72 kilogrammes).
The man's case showed that "self-prescription with complementary medicines can result in potentially harmful toxicities", and the problem may be more common than thought, researchers concluded.
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