Scientific evidence on the pros and cons of medical cannabis is limited, but interest is strong, judging by the sold-out turnout at the first symposium on the subject at the University of Hawaii Medical School.

Health professionals — including 52 physicians — made up most of the audience of 150 people at the educational event, according to organizer Amy Brown, associate professor of complementary and integrative medicine. A panel of doctors and lawyers shared highlights of research on the drug and regulations governing its use.

“Before starting a patient on a cannabinoid, you should be aware of what the pitfalls are, and one of the pitfalls is that there is terrible data,” said Dr. Thomas Murtaugh, a panelist and retired physician-researcher who has been involved in hundreds of clinical trials for a wide range of drugs.

“Nobody knows what kinds of benefits they are going to get versus what kind of side effects,” Murtaugh told attendees. “Be mindful. Explain to the patient. Start low and go slow. And be attentive.”

Although it remains illegal on the federal level, medical cannabis is available to registered patients through licensed dispensaries in Hawaii. Physicians may not legally prescribe it, but they can issue written certifications that a patient is qualified.

The delicate legal situation was underscored by a disclaimer at the Nov. 4 symposium: “John A. Burns School of Medicine is not recommending treatment with cannabis or stating that cannabis is capable of treating any medical condition.”

Marijuana is listed by the Drug Enforcement Administration as a Schedule 1 drug alongside heroin, in a category for drugs with “no currently accepted medical use” and “high potential for abuse.” As a result, research has been severely restricted in this country, although the drug was on the U.S. Pharmacopoiea list of medicinal drugs from 1854 until 1942.

With a majority of states now allowing medical use of cannabis, an expert committee of the National Academies of Sciences, Engineering and Medicine undertook a rigorous review of evidence and published a report on “The Health Effects of Cannabis and Cannabinoids” in January.

The report called for a broad national research agenda to delve into the subject. But based on studies to date, it found “conclusive or substantial” evidence that cannabinoids given orally are effective for chronic pain in adults, chemotherapy-induced nausea and MS-related muscle spasticicity.

On the other hand, cannabis use has been tied to falls and motor vehicle crashes, as well as overdose injuries among children in states where it is legal, according to the report.

Cannabis contains more than 100 different cannabinoids, chemical compounds with various effects. Two major ones are THC, the primary psychoactive compound, and CBD, or cannabidiol, which has nonpsychoactive effects.

Cannabis products vary widely in concentration, in the ratio of different cannabinoids and in methods of ingestion, from oral to smoking to tinctures.

“The concentration of THC has increased since the 1960s due to selective breeding of cannabis,” Brown said. “They basically upped the THC to high amounts.”

Dr. Tricia Wright, an associate professor in obstetrics and gynecology, called the the fetal brain “very vulnerable” and said that adolescents may also be at risk because their brains continue to mature into their 20s.

“In general, don’t recommend cannabis use to developing brains,” Wright said.

Murtaugh described the body’s own endocannabinoid system, which is involved in neural communication and maintaining equilibrium, and how it might interact with cannabis. People have varying amounts of two different kinds of cannabinoid receptors, he said: one that affects appetite, cognition, mood and nausea, and the other affecting the immune system, pain and inflammation. So their reactions to cannabis can differ widely.

He warned of certain conditions where its use can be problematic.

“If you have an active, acute illness, if you have active psychosis and if you have a history of arrhythmias, those three are contraindications for cannabinoids,” he said.

The national report found substantial evidence of a link between cannabis use and the development of schizophrenia, especially for heavy users who start early in adolescence.

“It is still unclear whether this means cannabis use precipitates schizophrenia or whether cannabis use is a form of self-medication, or due to other drugs such as amphetamines which heavy cannabis users are more likely to use,” said panelist Dr. David Roth, who is certified in pediatrics, psychiatry and neurology.

The national report is available at nationalacademies.org/CannabisHealthEffects.