A 67-year-old man in Australia got cyanide poisoning from the apricot kernel extract he was taking to beat cancer. The man didn’t die, but he had abnormally low levels of oxygen in his body — a side effect of cyanide poisoning that can be fatal.

Doctors in Melbourne realized the patient had low oxygen levels when he was under anesthesia for a routine surgery. Blood tests later revealed he had cyanide levels in his blood 25 times the accepted level. His case, described in the journal BMJ Case Reports, shows how alternative medications, including sham cancer treatments, can put patients at risk.

Many people believe that apricot kernels — the soft, almond-like seeds found inside apricot pits — can fight or prevent cancer. The 67-year-old in the case study had prostate cancer, and had been taking the apricot kernel extract, as well as a fruit kernel supplement called Novodalin, to prevent his prostate cancer from coming back, says study co-author Alex Konstantatos, the Head of Pain Medicine at Alfred Hospital in Melbourne, Australia.

Apricot kernels contain amygdalin, a compound also called laetrile, that is converted into cyanide in the body. Cyanide is toxic to cells because it interferes with their oxygen supply; it is particularly bad for the brain and heart, which require constant oxygen to function. People who believe in the power of apricot kernels think that this cyanide is only toxic to cancer cells, but scientists say that’s not true. “It is likely to kill all cells of the body equally,” Konstantatos writes in an email to The Verge. Laetrile doesn’t have any anticancer activity, according to the National Cancer Institute, and its use for treating cancer is illegal in the US.

Cyanide can cause nausea, headaches, insomnia, and nervousness, but it can also lead to death. In a report published last year, the European Food Safety Authority — Europe’s food safety watchdog — warned of many cases of children who were hospitalized because of cyanide poisoning caused by apricot kernels. A 28-month-old girl died because of high cyanide levels in her blood after eating 10 kernels.

The patient described in the case report was at the hospital for a routine surgery to check his bladder and urethra, called a cystoscopy. While he was under anesthesia, the doctors noticed that he had abnormally low levels of oxygen, called hypoxia. Because all cells need oxygen to function, hypoxia can have serious consequences, including death. So the doctors ordered some blood work. “The gentleman involved is very pleasant, intrigued and very inquisitive so he was happy to assist with our enquiries,” Konstantatos says.

25 times the accepted cyanide levels

The blood tests showed that the 67-year-old had high levels of thiocyanate, one of the byproducts of cyanide breakdown in the body. Thiocyanate is easier to measure than cyanide, so when the tests pointed to high thiocyanate levels, more blood was sent to a forensic laboratory near Sydney to confirm high levels of cyanide, says Konstantatos. The patient was found to have 25 times the accepted cyanide levels in his blood. That’s because he was ingesting two teaspoons of homemade apricot kernel extract and three tablets of Novodalin per day, equaling to 17.32 milligrams of cyanide.

Konstantatos says he wrote about this case study because he wanted to highlight that patients often take supplements and other “alternative medications” that can have serious health effects. But doctors often only ask about medically prescribed medications, and patients don’t often say that they’re taking supplements. “My message to my fellow doctors is to ask about these medications,” Konstantatos says.

As for the 67-year-old man, he was informed that the kernels were making him sick, but he decided to continue taking them. “He personally believes that the quality of evidence is sufficient for his purposes,” Konstantatos says, “or maybe he cannot wait for the scientific proof to come through as it may take too long to prevent his cancer from recurring.”