A compound in cannabis is 'significantly' effective in destroying cancerous tumours in leukaemia, research suggests.
A new study found that combining existing chemotherapy treatments with cannabinoids - the active chemicals in cannabis - had better results than chemotherapy alone.
The findings suggest that a lower dose of chemotherapy can be used on patients, minimising side effects of the treatment, say researchers.
Furthermore, scientists discovered that order the treatment was administered was crucial - using cannabinoids after chemotherapy resulted in a greater death of the blood cancer cells.
Lead author of the study, Dr Wai Liu, from St George's, University of London, said: 'We have shown for the first time that the order in which cannabinoids and chemotherapy are used is crucial in determining the overall effectiveness of this treatment.'
Last week, it was revealed singer Olivia Newton-John plans will use cannabis oil and 'other natural remedies' following her second breast cancer diagnosis.
According to her daughter, Chloe Lattanzi, the Grease star, 68, has opted to try the controversial substance in addition to modern medicine.
The medical value of cannabis has been hotly debated for years and there are many anecdotal cases reported of people claiming their cancer has been cured by using it.
A number of small studies have found that cannabis or cannabis derivatives may be helpful in treating nausea and vomiting from cancer chemotherapy, but research into its possible curative effects on the disease is in its infancy.
But according to Dr Wai Liu, cannabinoids are 'a very exciting prospect in oncology'.
Dr Wai Liu and his colleagues set out to examine the effects of different combinations of cannabinoids, known more specifically as phytocannabinoids, against leukaemia in the laboratory.
They wanted to test whether existing chemotherapy treatments worked effectively alongside the cannabinoids, and whether using the drugs in a different order had an effect.
In their report, published in the International Journal of Oncology, the team note that phytocannabinoids possess 'anticancer activity' when used alone in treatment.
And a number of these compounds have also been shown to combine favourably with each other to kill off leukaemia cells in laboratory tests, they said.
The researchers paired different types of cannabinoids and used them in combination with the common leukemia chemotherapy drugs cytarabine and vincristine.
They discovered using cannabinoids after chemotherapy resulted in greater induction of apoptosis - or death of the cancer cells.
The opposite was true when reversed and cannabinoids were given before chemotherapy.
Dr Wai Liu said: 'Studies such as ours serve to establish the best ways that they should be used to maximise a therapeutic effect.'
What the experts say
Cancer Research UK said it welcomes research into cannabinoids but said the results should be regarded with caution.
Senior science information manager Anna Perman said: 'This research in cells doesn't provide evidence that cannabinoids are safe or effective for patients.
'Researchers have been studying potential cancer-fighting chemicals found in cannabis for a while - but like any new treatment, these should only be used to treat patients once there's evidence that they improve outcomes.
'This is not to say that cannabinoids have no future role in cancer treatment, and Cancer Research UK supports clinical trials to treat cancer with cannabinoid drugs.
'But as it stands, we still need proper trials to know if they are effective, for what types of cancer, and at what dose.'
The researchers themselves say more trials need to be carried out to establish the voracity of their claims.
The charity warned cancer patients against using cannabinoids.
'This study looked at cannabinoids not cannabis - which is illegal and could interfere with other treatments given to patients.
'We advise against the use of cannabinoids or any medicine bought on the internet, as these are not regulated and may not be safe.'
Dr Wai Liu's team pointed out that when scientists use cannabis in the lab, they don't use the whole plant.
'These extracts are highly concentrated and purified, so smoking marijuana will not have a similar effect,' said Dr Wai Liu.