Cannabis smoke is less likely to cause cancer than tobacco smoke, a leading US expert says.
THC is one of 60 cannabinoids derived from the cannabis plant
Dr Robert Melamede, of the University of Colorado, said that, while chemically the two were similar, tobacco was more carcinogenic.
He said the difference was mainly due to nicotine in tobacco, whereas cannabis may inhibit cancer because of the presence of the chemical THC.
But health campaigners warned against complacency.
Cannabis remains the most commonly-used drug in the UK with one in 10 people using it in the last year, according to the British Crime Survey.
The Class C drug, which was downgraded in 2004, has already been linked to mental health problems and breathing difficulties.
But scientists are also exploring whether it can be used to treat a range of conditions, including multiple sclerosis and Alzheimer's disease.
Dr Melamede said whereas nicotine activated carcinogenic compounds, THC - one of 60 cannabinoids derived from the cannabis plant - had been shown to inhibit them in mice cells.
"Compounds found in cannabis have been shown to kill numerous cancer types including lung, breast, prostate, leukaemia, lymphoma and skin cancer."
But he said the effects of cannabis were complex as evidence also suggested low doses of THC could stimulate growth of lung cancer cells.
And he added the two could interact as cannabis was often smoked with tobacco.
"It is possible that as the cannabis-consuming population ages, the long-term consequences of smoking cannabis may become more similar to what is observed with tobacco.
"However, current knowledge does not suggest that cannabis smoke will have a carcinogenic potential comparable to that resulting from exposure to tobacco smoke."
Jean King, director of tobacco control at Cancer Research UK, said many of the studies that had looked at the link between cancer and cannabis had used purified cannabinoids.
"Results from such studies may not represent the overall effects of cannabis smoke, which contains more than 400 chemicals.
"Smoke from tobacco and cannabis contains many of the same carcinogens, and cell damage linked to lung cancer has been found in the lungs of chronic cannabis smokers."
And she added there should be no complacency as cannabis was often smoked with tobacco, which is responsible for a quarter of all cancer deaths in the UK.