“The team – based at Queen Mary’s School of Medicine and Dentistry in London – have followed up on their previous findings that the main active ingredient in cannabis, tetrahydrocannabinol (THC) has the potential to be used effectively against some forms of cancer. THC has previously been shown to attack cancer cells by interfering with important growth-processing pathways, however its mechanism of doing so has remained a mystery. Now, Dr Liu and his colleagues, using microarray technology – allowing them to simultaneously detect changes in more than 18,000 genes in cells treated with THC – have begun to uncover the existence of processes through which THC can kill cancer cells and potentially promote survival.”
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The active ingredient in marijuana cuts tumor growth in common lung cancer in half and significantly reduces the ability of the cancer to spread, say researchers at Harvard University who tested the chemical in both lab and mouse studies.

Delta-tetrahydrocannabinol (THC), inhibits EGF-induced growth and migration in epidermal growth factor receptor (EGFR) expressing non-small cell lung cancer cell lines. Lung cancers that over-express EGFR are usually highly aggressive and resistant to chemotherapy.
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The researchers tested the chemicals on different human prostate cancer cell lines grown in the lab, and found that they could slow down their growth and trigger cell death – Professor Ines Diaz-Laviada and her team at the University of Alcala in Madrid
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“We know that there are as many or more carcinogens and co-carcinogens in marijuana smoke as in cigarettes,” researcher Donald Tashkin, MD, of UCLA’s David Geffen School of Medicine tells WebMD. “But we did not find any evidence for an increase in cancer risk for even heavy marijuana smoking.” Carcinogens are substances that cause cancer.
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Cellular studies and even some studies in animal models suggest that THC has antitumor properties, either by encouraging the death of genetically damaged cells that can become cancerous or by restricting the development of the blood supply that feeds tumors, Tashkin tells WebMD.
In a review of the research published last fall, University of Colorado molecular biologist Robert Melamede, PhD, concluded that the THC in cannabis seems to lessen the tumor-promoting properties of marijuana smoke.
http://www.webmd.com/lung-cancer/news/20060523/pot-smoking-not-linked-to-lung-cancer?page=2