Times are changing for America’s most popular illicit drug.

Cannabis, more commonly known as marijuana, is still illegal under U.S. federal law.

However, more than half of all states have legalized cannabis for medicinal use, and eight of those states, along with the District of Columbia, allow recreational use, too.

Now a new report from the National Academies of Sciences, Engineering and Medicine provides a scientific review of the health impacts of cannabis, ranging from the drug’s effectiveness in treating pain and certain illnesses to its risks for causing diseases, mental disorders, and injuries.

The report’s authors note that, unlike alcohol and tobacco — which may have their own risks — there are no accepted standards to help individuals make choices about cannabis safety.

“We conducted an in-depth and broad review of the most recent research to establish firmly what the science says and to highlight areas that still need further examination,” said Dr. Marie McCormick, chair of the report committee, and professor at Harvard T.H. Chan School of Public Health, in a press release.

The committee considered more than 10,000 scientific abstracts for the review and reached nearly 100 conclusions.

Read more: Can marijuana ease MS symptoms? »

Cannabis as medicine

The term “cannabis” can refer to any of the organic components that come from the Cannabis sativa plant — such as marijuana, hemp, and cannabinoids.

And despite its patchwork legal status, cannabis is as popular as ever.

In a recent national survey cited in the report, more than 22 million Americans ages 12 or older said they used cannabis in the past 30 days.

The reasons people use cannabis vary.

About 90 percent of people surveyed said their use was primarily recreational.

About 10 percent said they used cannabis for medicinal purposes, while 36 percent reported mixed therapeutic and recreational use.

But for those who use cannabis as a medicine, the effects can be dramatic.

For example, the committee said they found conclusive evidence that some oral cannabinoids prevent the nausea and vomiting cancer patients often experience when receiving chemotherapy.

Cannabinoids — such as tetrahydrocannabinol (THC) — are the active chemical compounds in cannabis.

For people with multiple sclerosis (MS), there’s evidence that taking cannabinoids reduces muscle spasms.

The committee also found substantial evidence from randomized controlled trials — the gold standard of scientific research — and other studies that people who use cannabis for chronic pain have real improvements in their symptoms.

For other health conditions, the picture is unclear.

Some studies suggest cannabis might be used to treat a variety of other conditions, including sleep disorders and weight loss related to HIV and AIDS.

But the report’s authors noted that further, higher quality research would be needed to know for sure.

The committee also identified some conditions that cannabis does not seem to help, such as dementia and glaucoma.

“We’ve tried to give patients, physicians, and policy makers enough available evidence on what uses of cannabis have been found effective and not found to be effective,” Dr. Sean Hennessy, one of the report’s authors, and professor of epidemiology and systems pharmacology at the University of Pennsylvania's Perelman School of Medicine, told Healthline.

Read more: Legalization of marijuana doesn’t increase teen use »

Understanding the risks

Though it has medicinal uses, cannabis also carries certain risks — for some groups of people more than others.

The committee identified three primary points where the evidence of risk was strongest.

“These include that initiating cannabis at a young age is a risk factor for developing problematic cannabis use later in life,” Ziva Cooper, assistant professor of clinical neurobiology at Columbia University Medical Center, told Healthline.

Cannabis use disorder (CUD) is a psychiatric disorder diagnosed when an individual has significant personal, social, physical, or psychological distress related to cannabis.

McCormick noted during a webinar about the report that the committee can safely say adolescents should not be using cannabis.

The second point concerns prenatal care.

The committee found strong evidence that pregnant women who smoke cannabis increase the risk that their baby will be born with lower birth weight.

Cooper told Healthline the committee made a third conclusion that long-term cannabis smoking increases the risk for chronic breathing problems, like chronic cough and bronchitis.

There’s also evidence those lung issues might go away if an individual stops smoking cannabis.

And in a finding that may seem counterintuitive, the authors noted that smoking cannabis may not raise the risk of cancers linked to smoking tobacco, such as lung cancer and head and neck cancers, based on some good-to-fair quality studies.

However, the committee said the drug may be linked with a type of testicular cancer.

There may be more serious risks related to cannabis, but a dearth of research makes it hard to know.

McCormick told Healthline the committee was struck by the limited amount of evidence available to address these questions.

“There really were a lot of holes,” she said.

Read more: If marijuana is medicine, why can’t we buy it in pharmacies? »

Other risks, benefits

The committee looked at a broad range of other health issues related to cannabis, including the risk of being in a car accident, having a heart attack, or experiencing impairments to learning and memory, among others. 

Mental health is an area where the committee found a mix of possible risks and benefits.

On one hand, there’s substantial evidence that cannabis use raises the risk of developing schizophrenia, other psychoses, and social anxiety disorders.

But, on the other hand, there’s some evidence that cannabis may actually improve symptoms of post-traumatic stress disorder (PTSD) and Tourette’s syndrome.

Dr. Ken Duckworth, medical director for the National Alliance on Mental Illness (NAMI), who was not involved in the review, said there’s a real need for more research to understand those risks.

“People with a family history of psychosis typically don’t know that use of marijuana increases their risk. That’s an important public health failure,” Duckworth told Healthline.

Duckworth said he hopes to see more randomized controlled trials in the future that can help uncover the specific ingredients in cannabis that may help or harm mental health.

Read more: Colorado marijuana engineered to get you higher »

The future of research

The report’s authors noted there isn’t enough evidence to know how cannabis impacts many different areas of human health.

To address those gaps, they recommended greater funding for future studies and the development of a national cannabis research agenda. 

The committee also recommended that several federal agencies, along with public and industry groups, examine how legal barriers impact the study of cannabis.

Even though cannabis is legal for medical use in the majority of states, the federal government still classifies it as a “Schedule 1 drug.”

That classification indicates the drug has no medicinal value and has a high risk for abuse.

It also makes it hard for researchers to access the quality and quantity of cannabis products that are needed for certain kinds of research, according to Dr. Robert Wallace, one of the report’s authors, and a professor of epidemiology and internal medicine at University of Iowa College of Public Health.

“It was clear from interviews with many investigators that there are a group of regulatory barriers,” Wallace told Healthline. “A lot of this centers around the fact that cannabis is a Schedule 1 substance. That is the highest level of control. We’re hoping this can be addressed in some way.”