20 Years of Marijuana Research Summarized in New Report

Marijuana use has been linked to psychological dependence, impaired motor control and mental health problems

/ Author:  / Reviewed by: Joseph V. Madia, MD Beth Bolt, RPh

(RxWiki News) States across the US continue to debate the merits of allowing medicinal and, in some cases, recreational marijuana use. Yet there’s still uncertainty about the long-term health impacts of cannabis.

Researchers recently reviewed what the past two decades of research have revealed about the negative health effects of recreational marijuana use.

Those researchers found that studies have shown that marijuana use may increase the risk of car accidents and could lead to dependence.

The review also pointed to a possible link between regular marijuana use and mental health issues in adulthood.

Wayne Hall of the University of Queensland in Australia and colleagues looked at how medical research into the effects of marijuana has advanced in the last two decades.

The researchers started with studies and data from 1993. At that time, most research was from animal studies and limited human trials. That was compared to similar evidence from 2013.

Hall and team found that the risk of fatal cannabis overdose was “extremely small." This conclusion, according to Hall, is uncontroversial "because the dose of THC that kills rodents is extremely high,” he wrote.

THC, or tetrahydrocannabinol, is a chemical in marijuana that makes a user feel high.

The researchers also found evidence that marijuana caused negative effects such as feelings of anxiety or paranoia. Attention, memory and motor control problems were also observed. The researchers noted an increased risk of psychotic symptoms, especially in people with family history of mental health issues.

Psychotic symptoms are caused by a mental health condition called psychosis. Such symptoms include hallucinations and false beliefs about what is taking place.

On the link between marijuana use and car crashes, Hall wrote that scientific studies improved in the past decade. Hall and his team reviewed 26 relevant trials.

Hall wrote that recent marijuana users, based on THC blood level, were between 1.92 times and 2.21 times higher of getting into an accident compared to someone who did not use marijuana.

The researchers pegged the risk of developing dependence among people who have used marijuana at 9 percent in the United States.

The researchers also found regular marijuana-users had a 2.09 times higher risk of developing psychotic symptoms or disorders compared to non-marijuana users.

Hall and colleagues also examined the effects of regular marijuana use in young people.

“Regular adolescent cannabis users have lower educational attainment than non-using peers,” and are also more likely to try more illicit drugs, the authors wrote.

This review was published online Oct. 6 in Addiction.

The Australian National Health and Medical Research Council funded the research. The researchers disclosed no conflicts of interest.

Review Date: 
October 6, 2014
Last Updated:
October 7, 2014