If an American doctor of the late 19th century stepped into a time warp and emerged in 2010, he would be shocked by the multitude of pharmaceuticals that today's physicians use. But as he pondered this array (and wondered, as I do, whether most are really necessary), he would soon notice an equally surprising omission, and exclaim, "Where's my Cannabis indica?"

No wonder -- the poor fellow would feel nearly helpless without it. In his day, labor pains, asthma, nervous disorders and even colicky babies were treated with a fluid extract of Cannabis indica, also known as "Indian hemp." (Cannabis is generally seen as having three species -- sativa, indica and ruderalis -- but crossbreeding is common, especially between sativa and indica.) At least 100 scientific papers published in the 19th century backed up such uses.

Then the Marihuana Tax Act of 1937 made possession or transfer of Cannabis illegal in the U.S. except for certain medical and industrial uses, which were heavily taxed. The legislation began a long process of making Cannabis use illegal altogether. Many historians have examined this sorry chapter in American legislative history, and the dubious evidence for Cannabis addiction and violent behavior used to secure the bill's passage. "Under the Influence: The Disinformation Guide to Drugs" by Preston Peet makes a persuasive case that the Act's real purpose was to quash the hemp industry, making synthetic fibers more valuable for industrialists who owned the patents.

Meanwhile, as a medical doctor and botanist, my aim has always been to filter out the cultural noise surrounding the genus Cannabis and see it dispassionately: as a plant with bioactivity in human beings that may have therapeutic value. From this perspective, what can it offer us?

As it turns out, a great deal. Research into possible medical uses of Cannabis is enjoying a renaissance. In recent years, studies have shown potential for treating nausea, vomiting, premenstrual syndrome, insomnia, migraines, multiple sclerosis, spinal cord injuries, alcohol abuse, collagen-induced arthritis, asthma, atherosclerosis, bipolar disorder, depression, Huntington's disease, Parkinson's disease, sickle-cell disease, sleep apnea, Alzheimer's disease and anorexia nervosa.

But perhaps most exciting, cannabinoids (chemical constituents of Cannabis, the best known being tetrahydrocannabinol or THC) may have a primary role in cancer treatment and prevention. A number of studies have shown that these compounds can inhibit tumor growth in laboratory animal models. In part, this is achieved by inhibiting angiogenesis, the formation of new blood vessels that tumors need in order to grow. What's more, cannabinoids seem to kill tumor cells without affecting surrounding normal cells. If these findings hold true as research progresses, cannabinoids would demonstrate a huge advantage over conventional chemotherapy agents, which too often destroy normal cells as well as cancer cells.

As long ago as 1975, researchers reported that cannabinoids inhibited the growth of a certain type of lung cancer cell in test tubes and in mice. Since then, laboratory studies have shown that cannabinoids have effects against tumor cells from glioblastoma (a deadly type of brain cancer) as well as those from thyroid cancer¸ leukemia/lymphoma, and skin, uterus, breast, stomach, colorectal, pancreatic and prostate cancers.

So far, the only human test of cannabinoids against cancer was performed in Spain, and was designed to determine if treatment was safe, not whether it was effective. (In studies on humans, such "phase one trials," are focused on establishing the safety of a new drug, as well as the right dosage.) In the Spanish study, reported in 2006, the dose was administered intracranially, directly into the tumors of patients with recurrent brain cancer. The investigation established the safety of the dose and showed that the compound used decreased cell proliferation in at least two of nine patients studied.

It is not clear that smoking marijuana achieves blood levels high enough to have these anticancer effects. We need more human research, including well-designed studies to find the best mode of administration.

If you want to learn more about this subject, I recommend an excellent documentary film, "What If Cannabis Cured Cancer," by Len Richmond, which summarizes the remarkable research findings of recent years. Most medical doctors are not aware of this information and its implications for cancer prevention and treatment. The film presents compelling evidence that our current policy on Cannabis is counterproductive.

Another reliable source of information is the chapter on cannabinoids and cancer in "Integrative Oncology" (Oxford University Press, 2009), a textbook I edited with integrative oncologist Donald I. Abrams, M.D. (Learn more about integrative cancer treatment from Dr. Abrams.)

After more than 70 years of misinformation about this botanical remedy, I am delighted that we are finally gaining a mature understanding of its immense therapeutic potential.