Marijuana and cannabis can help to safely alleviate pain for some patients but many uncertainties remain about their health and safety risks, according to a major scientific review published Thursday.
A committee of the US National Academies of Sciences, Engineering, and Medicine looked at more than 10 000 scientific abstracts to reach its nearly 100 conclusions.
The report by a 17-member panel offers a "rigorous review of relevant scientific research published since 1999," said a statement from NASEM.
Cannabis, the scientific name for the plant whose dried buds and leaves make marijuana, is the most popular illicit drug in the United States.
A recent nationwide survey showed more than 22 million Americans ages 12 and older reported using the drug in the past 30 days.
Nine in 10 adult users said it was for recreational purposes, with just 10 percent saying it was solely for medical purposes.
"For years the landscape of marijuana use has been rapidly shifting as more and more states are legalizing cannabis for the treatment of medical conditions and recreational use," said Marie McCormick, chair of the committee and a professor of maternal and child health at Harvard University.
"This growing acceptance, accessibility, and use of cannabis and its derivatives have raised important public health concerns."
She said the report aims to address the fact that the "lack of any aggregated knowledge of cannabis-related health effects has led to uncertainty about what, if any, are the harms or benefits from its use."
The report found that patients who used cannabis to treat chronic pain were "more likely to experience a significant reduction in pain symptoms."
Adults with multiple sclerosis-related muscle spasms also improved their symptoms when using certain "oral cannabinoids" -- or man-made, cannabinoid-based medications.
Conclusive evidence was also found that these oral cannabinoids could prevent and treat nausea and vomiting in people with cancer undergoing chemotherapy.
"Smoking cannabis does not increase the risk for cancers often associated with tobacco use -- such as lung and head and neck cancers," added the report.
The committee "found limited evidence that cannabis use is associated with one sub-type of testicular cancer."
Risks of cannabis use however include the possibility of triggering a heart attack, but more research is needed to understand "whether and how cannabis use is associated with heart attack, stroke, and diabetes."
Smoking cannabis can lead to bronchitis and chronic cough, but "quitting cannabis smoking is likely to reduce these conditions" and it remains unclear if there is any link to respiratory diseases, including chronic obstructive pulmonary disease, asthma, or worsened lung function.
When it comes to mental health, the committee found that "cannabis use is likely to increase the risk of developing schizophrenia, other psychoses, and social anxiety disorders, and to a lesser extent depression."
Thoughts of suicide can increase among heavy cannabis users, and may make bipolar disorder symptoms worse.
But in people with schizophrenia and other psychoses, "a history of cannabis use may be linked to better performance on learning and memory tasks," it said.
The more people use cannabis -- and the younger they start -- the more likely they are to develop a what is known as "problem cannabis use."
But the committee found limited evidence that cannabis use increases the rate of initiating other drug use.
Even if brain functions such as learning, memory, and attention are impaired after using cannabis, researchers found few signs of long-term damage in people who have stopped smoking it.
In pregnant women, some evidence has shown that smoking cannabis during pregnancy is linked to lower birth weight, but the relationship with other pregnancy and childhood outcomes is unclear.
Accidents among children, including ingesting cannabis, have risen sharply since the substance was legalized in some parts of the United States.
Not surprisingly. the report found "substantial evidence" that cannabis use is linked to impaired driving and vehicle crashes.
Advocates of medical marijuana said the report showed that cannabis can help people, and urged the federal government to legalize it.
Marijuana is currently a Schedule I substance under the Controlled Substances Act, which means it is determined to have has no medical value.
"This report is vindication for all the many researchers, patients and healthcare providers who have long understood the benefits of medical marijuana," said Michael Collins, deputy director of national affairs at the Drug Policy Alliance.
Paul Armentano, deputy director of NORML, a lobbying group that aims to legalize marijuana, pointed out that the evidence is not new.
"Yet for decades marijuana policy in this country has largely been driven by rhetoric and emotion, not science and evidence," he said.
"At a minimum, we know enough about cannabis, as well as the failures of cannabis prohibition, to regulate its consumption by adults, end its longstanding criminalization, and to remove it from its Schedule I prohibitive under federal law."