Opinions vary widely among scientists and doctors about whether marijuana is effective as medicine. But nearly everyone agrees there is enough anecdotal evidence, and enough promise from studies on animals, that it should be studied further on people, using rigorous scientific standards that can be published in reputable medical journals and reviewed by other scientists.
Most of the medical literature on marijuana focuses on addiction. Most of those studies are funded by drug companies developing medications to treat withdrawal.
But there are some accepted studies that measure the drug as medicine. Most of the studies are first steps in research, using small samples.
According to the National Cancer Institute (NCI), several scientific studies on animals have shown promise for the chemicals called cannabinoids in marijuana that cause drug-like effects for treating cancer tumors.
“Studies in mice and rats have shown that cannabinoids may inhibit tumor growth by causing cell death, blocking cell growth, and blocking the development of blood vessels needed by tumors to grow,” according to NCI. “Laboratory and animal studies have shown that cannabinoids may be able to kill cancer cells while protecting normal cells.”
There are no studies on humans to determine whether marijuana is an effective treatment for cancer tumors.
Proponents of medical marijuana say it helps alleviate nausea associated with chemotherapy, but the NCI says results of small clinical studies are inconclusive.
“Three small trials have studied inhaled cannabis for the treatment of chemotherapy-related nausea and vomiting,” according to NCI. “There is not enough information to interpret these findings.”
Some scientists say further study is unwarranted, because the newest drugs prescribed for nausea are very effective.
Several studies focus on marijuana as a medicine to alleviate pain caused by conditions or injuries that affect the nervous system, called neuropathic pain by researchers.
In a 2007 study of neuropathic pain related to HIV, by researchers at the University of California, San Francisco, 50 patients smoked marijuana in a closely controlled experiment. According to the study, smoked marijuana reduced pain by 34 percent in the patients who used it, compared to a 17 percent reduction of pain in a control group that got a placebo.
“The study concluded that a significantly greater proportion of patients who smoked cannabis (52 percent) had a greater than 30 percent reduction in pain intensity, compared to only 24 percent in the placebo group,” the authors of the report wrote. “This result is clinically important since the threshold of a 30 percent reduction in pain intensity is associated with meaningful improvement in quality of life.”
Another study of 30 patients, done at the University of California, San Diego, measured marijuana as a treatment for the chronic, painful contraction of muscles, called spasticity, experienced by multiple sclerosis patients.
“Compared to placebo cigarettes, cannabis was found to significantly reduce both an objective measure of spasticity, and pain intensity,” the study’s authors concluded, “and provided some benefit beyond currently prescribed treatments.”
Another experiment on animals indicated marijuana did not suppress fear-inducing memories, a study that suggests marijuana may not be an effective treatment for anxiety disorders, including Post-Tramuatic Stress Disorder (PTSD).
Long term effect
A recent study that concluded that persistent marijuana use by adolescents is harmful has generated much discussion in the medical and scientific community.
In a long-term study, more than 1,000 people were closely tracked from birth to age 38. Using data from that study, the National Academy of Scientists concluded that people who began using marijuana as adolescents suffered significant problems later in life.
“Persistent cannabis use was associated with neuropsychological decline broadly across domains of functioning, even after controlling for years of education,” the study’s authors wrote. “Impairment was concentrated among adolescent-onset cannabis users, with more persistent use associated with greater decline.”
The authors also concluded that the declines were not reversed when people stopped using marijuana.
Researchers at the University of Oxford in England, using the same data, came to a different conclusion. They concluded that differences in social and economic status over time could account for a decline in intelligence in the people studied.
