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On its website The National Cancer Institute, part of the US department of health, said: “Laboratory and animal studies have shown that cannabinoids (the active ingredient in cannabis) may be able to kill cancer cells while protecting normal cells.
“They may inhibit tumour growth by causing cell death, blocking cell growth, and blocking the development of blood vessels needed by tumours to grow.”
The studies in rodents show that cannabinoids may reduce the risk of colon, liver and breast cancer, and could make chemotherapy more effective.
But researchers added: “At this time, there is not enough evidence to recommend that patients inhale or ingest cannabis as a treatment for cancer-related symptoms or side effects of cancer therapy.”
In many US states where cannabis is already legal for medicinal use, cancer patients have long been using the drug to ease pain.
The Cancer Research charity reacted cautiously, saying there was no evidence of a similar effect in humans.
A spokesman said: “There isn’t enough reliable evidence to prove that cannabinoids, whether natural or synthetic, can effectively treat cancer in patients, although research is ongoing around the world.”
The charity has also warned patients to be wary of fraudsters selling cannabis treatments.