Humans have cultivated marijuana for thousands of years — using fiber for cloth, oil for food, smoke for ceremonies and chemicals for medicine.
Scientists know that compounds found in pot can alter physical and mental functions, lending credence to sick people who say marijuana alleviates their symptoms.
But marijuana — especially in its smoked form — carries risk.
Some studies have linked heavy pot use to heart attacks, mental illness and breathing difficulties.
"Marijuana is not a completely benign substance. It is a powerful drug with a variety of effects,'' concluded the Institute of Medicine, a neutral research group charged by Congress to study medical marijuana
On Nov. 4, Floridians will vote on legalizing marijuana for medical use. Supporters point to sympathetic patients who say they need it.
Possible negative health effects are harder to decipher, because studies often conflict. But grounds for caution do exist.
Every medicine carries potentially harmful side effects — listen to the warnings at the end of any prescription drug ad on TV.
If marijuana were a prescription drug, it would have undergone the gold standard clinical trials that FDA approval requires. Doctors and patients could evaluate doses and consider benefit versus risk.
But marijuana is an unregulated herbal remedy. Different strains have different chemical content. People smoke it, eat it or take it in liquid form. Some users imbibe heavily for years, others just occasionally.
Plus, the federal government dampens research by labeling pot as an illegal substance with no medical value.
Still, hundreds of small studies have examined various forms of pot in limited fashion, some with potentially negative findings:
The National Cancer Institute warns that a few studies suggest that marijuana might promote some cancers.
Of particular concern are the carcinogenic tars in smoked pot. They are stronger than those in cigarettes. Usage is hard to measure because pot smokers inhale less than cigarette users do but hold smoke down longer.
So far, few studies have shown elevated risk of lung cancer with pot use — once scientists separate out people who also smoke cigarettes.
Kaiser Permanente, a huge West Coast HMO, followed 65,000 members for an average of eight years and found no increased cancer risk with pot use — with one exception: Prostate cancer rose among young pot users, though it remained very rare.
The Kaiser study, though reassuring, had its limits. It followed men between the ages of 15 and 49 — younger than the ages when many cancers typically appear. Many HMO members smoked marijuana only occasionally, which might have masked bad outcomes in those who smoked more intensely, the Institute of Medicine reported.
"Despite the large cohort size, there might not have been a sufficient number of heavy or long-term marijuana users to reveal an effect,'' the report said.
Vaporizing — which uses heat to release useful chemicals without burning the plant material — could reduce lung cancer risk, as could eating pot, Canada's health agency advised. But to put the lung cancer fear completely to rest, the agency said, "further well-controlled epidemiological studies are required.''
One pot-related malignancy has shown up: The risk of testicular cancer in young pot users appears to double from about six cases in 100,000 users to 12.
Complicating the cancer equation is the possible protective effect of cannabis. The National Cancer Institute cited rat and lab dish studies where tumors exposed to cannabinoid chemicals failed to spread as expected and even shrank. Pot also helped prescription drugs work their way into cancerous cells.
In one survey of nine epidemiological studies, the National Cancer Institute said, throat cancers seemed to increase among pot smokers while tongue cancers decreased.
"Cannabinoids could provide unquestionable advantages compared to current antitumoral therapies,'' the biotechnology branch of the National Institutes of Health reports.
People who smoke pot daily or almost daily are likely to have respiratory problems, similar to those experienced by cigarette smokers, the NIH's biotechnology division says.
In one study, heavy marijuana smokers were two to three times more likely to suffer from bronchitis than a nonuser, and five to seven times more likely to have a persistent cough.
Evidence is slim that marijuana use — by itself — contributes to chronic obstructive pulmonary disease. But a study reported by the NIH found that cigarette smokers increase their chances of COPD if they also smoke pot.
The heart beats faster during marijuana use and for at least 20 to 30 minutes afterward.
Blood pressure may decrease at high doses, particularly when users stand up, the Institute of Medicine reported. But some studies show that pressure can increase when people lie down.
A 2001 study indicated that the chance of a heart attack increases 300 to 500 percent for about an hour after smoking pot — somewhat akin to the stress of vigorous exercise. Other things being equal, people who smoke a joint every day will increase their chance of a heart attack over the course of a year by 1 to 3 percent, the study indicated. That compares to cigarette smoking, which causes up to 30 percent of heart disease deaths in the United States.
Cardiovascular changes "have not posed a health problem for healthy, young users,'' the Institute of Medicine wrote. "However, such changes in heart rate and blood pressure could present a serious problem for older patients.''
Most strains of marijuana are psychoactive — capable of causing anxiety, hallucinations and even paranoia, particularly in novices. Short-term memory can suffer, as can coordination.
Such symptoms abate after use stops, but studies have raised concerns about mental health.
Some users report that pot calms them, yet regular use is associated with depression and anxiety in others. People with schizophrenia who smoke pot seem to have earlier onset and more frequent episodes.
Marijuana stimulates production of dopamine, a neurotransmitter that affects mood. PET scans show heavy use is associated with structural changes in the brain. What's less clear is causality. Maybe depression and anxiety lead to pot-smoking in some users, rather than the reverse.
English researchers recently discovered a gene that makes people more likely to have schizophrenia but also more likely to smoke pot — offering a new explanation for the link between higher pot use and troubling schizophrenia symptoms. Maybe the gene, not pot, is the main culprit.
The Institute of Medicine recommended clinical trials to sort out the mental health effects of cannabis —no easy task. Changes in the brain "are potentially undesirable in some patients and situations and beneficial in others,'' the institute concluded.
The strongest current evidence about physical, social and psychological dangers of marijuana relates to adolescents, whose brains are still developing. There is evidence that early pot use can alter the brain's pleasure and reward system, making young people more susceptible to abuse of opiate-based drugs.
But the relevance of adolescent drug use to a medical marijuana debate is unclear. In national surveys, teens report that illegal marijuana is easy to acquire, regardless of whether medical use is permitted. Several studies have indicated that medical marijuana laws do not lead to higher teen use. No significant studies have shown that they do.
Contact Stephen Nohlgren at email@example.com.