A 67-year-old man who took apricot kernel extract believing it would improve his health found that it gave him cyanide poisoning instead.

Despite the diagnosis, the man told his doctors he planned to keep on taking the supplements.

Australian doctors describe the man’s case in the journal BMJ Case Reports. They say the otherwise healthy man came into hospital for a routine procedure called a cystoscopy, in which doctors use a scope to examine the lining of the bladder.

When the man was under anesthetic for the procedure, doctors noticed he had abnormally low levels of oxygen in his body, despite their attempts to deliver oxygen to him.

After he underwent heart tests and chest X-rays, blood tests revealed the problem: he had extremely high levels of cyanide in his blood.

While a normal amount of cyanide is somewhere between 20 and 80 μmol/L for non-smokers, this man had 521μmol/L.

The report authors say that based on how much seed extracts the man was taking, he was ingesting more than 17 mg of cyanide every day, “which is enough to raise blood cyanide to around 25 times above acceptable levels.”

The man, who was in remission from prostate cancer, explained that he had been taking two teaspoons of homemade apricot kernel extract every day for the past five years, as well as three tablets of Novodalin, a herbal fruit kernel supplement.

Apricot kernels, or seeds, contain amygdalin, also called laetrile or vitamin B17. Some alternative health advocatesbelieve it has cancer-fighting properties and refute concerns that amygdalin can be converted into cyanide.

Health Canada says there is no evidence that amygdalin can prevent cancer, and it is illegal for companies to make any claim that the supplement can cure or prevent any medical condition, including cancer.

The man stopped taking the supplement for three days and his blood oxygen levels quickly returned to normal.

But the doctors report he went right back to using it.

“Despite conveying our concerns regarding the continued ingestion of ake (apricot kernel extract) and being satisfied that our viewpoint was understood, the patient elected to continue self-administering ake,” they write.

The doctors say the case illustrates how long-term use of self-prescribed “complementary medicine” can result in harmful poisoning, “which may carry potential for serious consequences.”