Expert Voices

Is Marijuana Harmful? (Op-Ed)

a marijuana joint
(Image credit: Amihays | Shutterstock)

Jeff Nesbit was the director of public affairs for two prominent federal science agencies. This article was adapted from one that first appeared in U.S. News & World Report. Nesbit contributed the article to Live Science's Expert Voices: Op-Ed & Insights.

As more states consider decriminalizing marijuana, the scientific and public health communities are beginning to catch up with answers to some of the tough research questions about broad usage of the drug in the general population.

Is marijuana use addictive or habit-forming? Does it impair your ability to drive a car? Is it a gateway to the use of other illegal drugs? Does regular use increase the risk of cognitive impairment in adolescents?

A new paper in the journal Addiction (published by the Society for the Study of Addiction) has some definitive answers to these questions, based on a review of peer-reviewed research in adolescent and adult populations since 1993. The results, however, are not necessarily welcome ones for advocates of broad decriminalization of marijuana .

Marijuana's health effects

The paper, by lead researcher Wayne Hall from the University of Queensland Centre for Youth Substance Abuse Research, examined changes in the evidence on the adverse health effects of cannabis from 1993 to 2013. In effect, the study looked at health outcomes in 1993, and compared them to the same health outcomes 20 years later. It found that:

  • Research in the past 20 years  has shown that driving while cannabis-impaired approximately doubles car crash risk;
  • Around one in 10 regular cannabis users develop dependence;
  • Regular cannabis use in adolescence doubles the risks of leaving school early (prior to high school graduation);
  • Regular use of cannabis doubles the risk of cognitive impairment in adulthood. 

That second point — that regular marijuana use is addictive to about 10 percent of the adolescent population — has also been borne out in other recent studies, as well. It’s a worrisome effect for health officials. 

The paper also found that regular use of marijuana in adolescents — again, based on comparing outcomes in 1993 to the same set of outcomes 20 years later — is also "associated strongly" with the use of other illicit drugs. This has been a hotly debated subject for years, and this paper will likely exacerbate that debate.

The researchers also found that all of the negative outcomes emerge from the research data even after scientists controlled for other variables beyond the simple question of marijuana use.

"This suggests that cannabis use is a contributory cause of these outcomes, but some researchers still argue that these relationships are explained by shared causes or risk factors," Hall wrote in the abstract.

The research paper also identified other potential risks for regularly smoking marijuana.

"Cannabis smoking probably increases cardiovascular disease risk in middle-aged adults, but its effects on respiratory function and respiratory cancer remain unclear, because most cannabis smokers have smoked, or still smoke, tobacco," Hall wrote in the abstract.

Marijuana has changed, as have its effects

Despite repeated efforts to impose criminal penalties on the use and sale of marijuana, regular cannabis use among adolescents and young adults is now almost as common as tobacco use. [11 Odd Facts About Marijuana ]

What has changed in the past generation is the amount of tetrahydrocannabinol (THC) — the psychoactive ingredient in pot that produces effects of euphoria and increased sociability in users — has increased in recreational marijuana.

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During the past 30 years, the THC content of cannabis consumed in the United States has increased from less than two percent of a typical cannabis plant extract to 8.5 percent in 2006 — more than a fourfold increase in the percentage level of the psychoactive ingredient.

Those two factors — widespread use in the adolescent population, combined with a significant increase in the psychoactive ingredient — is what health researchers during the past 20 years have focused-in on as marijuana use has become widespread. "The impacts of increased potency on cannabis use should be a research priority," Hall argues.

What the paper does, in a dry and methodical manner, is explain how much researchers and physicians have learned about widespread use of marijuana by adolescents since 1993. What they knew 20 years ago was largely confined to studies of the effects of THC in animal studies. Now, 20 years later, they have lots and lots of data from studies that tracked outcomes in people.

Marijuana is now a public-health issue

From that research, the health community is now fairly certain about the a long list of adverse effects when it comes to marijuana use. Some of it is reassuring — like the fact that cannabis use does not produce fatal overdoses as do opioids (such as prescription pain medications).

But the risk of impaired driving, especially when combined with alcohol, and long-term health effects from chronic use, like dependency and cognitive impairment, are real public-health questions that need to be considered and addressed.

These questions about the adverse health impacts are especially critical right now, given the fact that marijuana decriminalization efforts in states across the United States are likely to be successful, boosting even wider use in adolescents and young adults.

Baseline health research has been critical in understanding the effects of short-term and chronic alcohol usage once a legal drinking age was established in the United States. We should demand nothing less for marijuana use.

This Op-Ed was adapted from one that first appeared in Nesbit's column At the Edge in U.S. News & World Report. Nesbit's most recent Op-Ed was "Sexism and Science Go Hand-in-Hand." Follow all of the Expert Voices issues and debates — and become part of the discussion — on Facebook, Twitter and Google+. The views expressed are those of the author and do not necessarily reflect the views of the publisher. This version of the article was originally published on Live Science.