Doctors have issued a warning about the dangers of taking unsubstantiated natural health remedies, after a Victorian man accidentally gave himself cyanide poisoning.

According to a report published in the BMJ medical journal, the unnamed 67-year-old went to hospital for some routine surgery in 2015 when doctors noticed he had low levels of oxygen in his red blood cells.

The only trouble is, they couldn't figure out why.

He appeared to be fit and healthy, and he told doctors that he didn't smoke, news.com.au reports.

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According to HuffPost, he also cycled 80 kilometres per week.

Eventually, he told doctors he'd taken two teaspoons of homemade apricot kernel extract and three herbal fruit kernel supplements daily for five years.

He was in remission from prostate cancer, and apparently he'd read somewhere apricot kernels could stop the disease from returning.

However, the seeds are known to contain a substance called amygdalin, a poisonous compound the body turns into cyanide.

It soon became clear the patient had been poisoning himself.

"(The patient) believed very strongly that the kernel extract was going to prevent a cancer... from coming back," Dr Konstantatos told AAP.

"It's not known what the effect of having a level of cyanide that's not enough to kill you suddenly, but (is) still quite high - we don't know what the long term effects of it are."

He said although cyanide is marketed for cancer prevention, there's no proof it helps.

It echoed a warning from the Cancer Council in 2015, when they said apricot seeds were ineffective at treating the disease and also very dangerous.

The raw kernels were actually banned from sale in Australia that year.

"There have been a number of cases of cyanide poisoning related to consumption of apricot kernels, with some consumers eating them believing they can help cure or prevent cancer, although there is no credible evidence that is the case," former Food Standards Australia New Zealand boss Steve McCutcheon said at the time.

Natural medicine is a booming in popularity.

In February this year, News Corp reported Australian spending on supplements had grown by 11 per cent over five years, creating a $1.85 billion industry.

However, Dr Konstantatos said this case highlighted a broad problem.

"We went to all the trouble to measure it, confirm it and to publish the study to show that this gentleman's probably just the tip of the iceberg," he said.

"People who take medicines like this, in this unregulated way, do not know how much, exactly, they're taking.

"That's exactly what makes it so dangerous."

- with Georgie Moore from AAP