Cannabis smoke is not as carcinogenic as tobacco smoke. In a review article published today in Harm Reduction Journal, Dr. Melamede from the University of Colorado, Colorado Springs, USA, writes that although cannabis smoke and tobacco smoke are chemically very similar, evidence suggests that their effects are very different and that cannabis smoke is less carcinogenic than tobacco smoke.

The pharmacological effects of tobacco and cannabis smoke differ in many ways, mainly because tobacco smoke contains nicotine while cannabis smoke contains tetrahydrocannabinol (THC). The cancer-promoting effects of smoke are increased by nicotine, while they are reduced by THC.

Tobacco and cannabis smoke contain the same carcinogenic compounds - and depending on which part of the plant is smoked, cannabis smoke can contain more of them - but, whereas nicotine activates these carcinogenic compounds, THC has been shown to inhibit them in mice cells. THC is very likely to have protective effects against the carcinogens present in smoke in humans too, but cannabis smoke remains nonetheless carcinogenic.

While nicotine and THC can act on related cellular pathways, they bind to different receptors to activate these pathways. The cells of lungs and the respiratory passages are lined with nicotine receptors, but do not appear to carry THC receptors. This explains why cannabis smoking has not been associated with lung cancer, a main cause of death from cigarette smoking.

Cannabis has also been shown to kill cancer cells and to reduce tumour growth, in part by reducing the formation of blood vessels that feed tumours. But "the effects of cannabinoids are complex and sometimes contradicting", warns the author. In addition, as cannabis is frequently smoked with tobacco, the effects of the two drugs may interact in complex ways.

Cannabis is a class C drug in the UK and the USA. It has been linked to an increased risk of psychosis and schizophrenia, in a small group of vulnerable individuals. But there is increasing evidence that the drug has significant medicinal uses and can greatly improve the lives of patients suffering from a wide range of conditions, including multiple sclerosis, AIDS, Alzheimer's disease and insomnia. In spite of this, governments have been reluctant to legalize cannabis for medical use, on the grounds that the risks associated with the drug still outweigh its benefits.

Cannabis and Tobacco Smoke are not Equally Carcinogenic
Robert J Melamede
Harm Reduction Journal, in press

Juliette Savin
BioMed Central