"BBC News
October 17, 2005

Cannabis smoke is less likely to cause cancer than tobacco smoke, a leading US expert says.

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.

"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" Jean King, of Cancer Research UK

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."