Cristina Sanchez, a young biologist at Complutense University in Madrid, was studying cell metabolism when she noticed something peculiar. She had been screening  brain cancer cells because they grow faster than normal cell lines and thus are useful for research purposes. But the  cancer cells died each time they were exposed to tetrahydrocannabinol (THC), the principal psychoactive ingredient of marijuana.

Instead of gaining insight into how cells function, Sanchez had stumbled upon the anti-cancer properties of THC. In 1998, she  reported in a European biochemistry journal that THC “induces apoptosis [cell death] in C6 glioma cells,” an aggressive form of brain cancer.

Subsequent peer-reviewed studies in several countries would show that THC and other marijuana-derived compounds, known as “cannabinoids,” are effective not only for cancer-symptom management (nausea, pain, loss of appetite, fatigue),  they also confer a direct antitumoral effect .

A team of Spanish scientists led by Manuel Guzman conducted the first clinical trial assessing the antitumoral action of THC on human beings. Guzman administered pure THC via a catheter into the tumors of nine hospitalized patients with glioblastoma, who had failed to respond to standard brain-cancer therapies. The results were published in 2006 in the  British Journal of Pharmacology : THC treatment was associated with significantly reduced tumor cell proliferation in  every test subject .

Around the same time, Harvard University scientists ++reported++[ http://www.nature.com/bjc/journal/v95/n2/abs/6603236a.html ] that THC slows tumor growth in common lung cancer and “significantly reduces the ability of the cancer to spread.” What’s more, like a heat-seeking missile, THC selectively targets and destroys tumor cells while leaving healthy cells unscathed. Conventional chemotherapy drugs, by contrast, are highly toxic; they indiscriminately damage the brain and body.

There is mounting evidence, according to a report in  Mini-Reviews in Medicinal Chemistry , that cannabinoids “represent a new class of  anticancer drugs that retard cancer growth, inhibit angiogenesis [the formation of new blood cells that feed a tumor] and the metastatic spreading of cancer cells.”

Dr. Sean McAllister, a scientist at the Pacific Medical Center in San Francisco, has been studying cannabinoid compounds for 10 years in a quest to develop new therapeutic interventions for various cancers. Backed by grants from the National Institute of Health (and with a license from the DEA), McAllister  discovered that cannabidiol (CBD), a nonpsychoactive component of the marijuana plant, is a potent inhibitor of breast cancer cell proliferation, metastasis, and tumor growth.

In 2007, McAllister published a  detailed account of how cannabidiol kills breast cancer cells and destroys malignant tumors by switching off expression of the ID-1 gene, a protein that appears to play a major role as a cancer cell conductor.

The ID-1 gene is active during human embryonic development, after which it turns off and stays off. But in breast cancer and several other types of metastatic cancer, the ID-1 gene becomes active again, causing malignant cells to invade and metastasize. “Dozens of aggressive cancers express this gene,” explains McAllister. He postulates that CBD, by virtue of its ability to silence ID-1 expression, could be a breakthrough anti-cancer medication.

“Cannabidiol offers hope of a non-toxic therapy that could treat aggressive forms of cancer without any of the painful side effects of chemotherapy,” says McAllister, who is seeking support to conduct clinical trials with the  marijuana compound on breast cancer patients.

McAllister’s lab also is analyzing how CBD works in combination with first-line chemotherapy agents. His research shows that cannabidiol, a potent antitumoral compound in its own right, acts synergistically with various anti-cancer pharmaceuticals, enhancing their impact while cutting the toxic dosage necessary for maximum effect.

Investigators at St. George’s University in London observed a similar pattern with THC, which magnified the effectiveness of conventional antileukemia therapies in preclinical studies. THC and cannabidiol both induce apoptosis in leukemic cell lines.

At the annual summer conference of the International Cannabinoid Research Society, held this year in Freiburg, Germany, 300 scientists from around the world discussed their latest findings, which are pointing the way toward novel treatment strategies for cancer and other degenerative diseases. Italian investigators described CBD as “the most efficacious inducer of apoptosis” in prostate cancer. Ditto for cannabidiol and colon cancer, according to British researchers at Lancaster University.

Within the medical science community, the discovery that cannabinoids have anti-tumoral properties is increasingly recognized as a seminal advancement in cancer therapeutics.

Martin A. Lee is the author of  Smoke Signals: A Social History of Marijuana – Medical, Recreational and Scientific (Scribner, August 2012). He is the cofounder of the media watch group FAIR, director of Project CBD, and the author of  Acid Dreams and  The Beast Reawakens . For more information and regular updates, follow Smoke Signals—the book on  Facebook .