Patients are buying an experimental cancer drug over the internet, it has been reported.
The drug has not yet been tested on humans as a cancer treatment
The drug, called DCA, has been shown to shrink tumours in rats but tests on humans are years away.
However Nature magazine reports some terminally ill patients are taking the drug because they do not want to wait for the research.
UK cancer experts warned patients there was no evidence DCA was beneficial, and said it could cause harm.
DCA, or dichloroacetate, is a small molecule that blocks an enzyme in mitochondria - which generate energy in cells and also control cell suicide.
Dr Evangelos Michelakis, of the University of Alberta in Canada, who has been investigating the drug, found the cancer cells turned off the suicide switch.
Tests he carried out on rats showed DCA can reignite the switch, and prompt the cells to die.
Tumours shrank by around 75% within three weeks.
Other work has shown DCA can kill human cancer cells in the lab, but no patients have actually been given the drug to treat their cancer.
Nature suggests up to 200 people around the world have bought the drug over the web.
But Dr Michelakis and others are concerned.
He said that the patients' use of the drug could undermine efforts to carry out a controlled trial if harmful side effects emerged and the drug earns a bad reputation.
"It's destroying efforts to do this right. Any way you look at this, it's a negative development."
The US Food and Drug Administration is investigating the web purchasing of DCA.
Experts say terminally ill patients want to try whatever is available, because they feel they have nothing to lose.
But Professor John Toy, medical director of Cancer Research UK,said he "strongly advised" patients not to buy DCA.
"DCA has not been tested for this purpose. DCA might not be helpful and, indeed, might be harmful when given to cancer patients."
He added: "It is understandable that people with cancer will want to try almost anything to treat their disease.
"However, there is no evidence to support the use of DCA for treating cancer. Patients would be wise to continue consulting their specialist for advice."