Pot's increasing potency could make it more likely that toking will interfere with users' lives, a new study argues.
Compared to pot of the 1990s, today's marijuana contains significantly higher levels of THC, the chemical compound that causes intoxication, the research team notes.
This added punch may be associated with an higher risk of cannabis use disorder, researchers from the University of Michigan and Brown University conclude.
They said the THC concentration of pot confiscated by the U.S. Drug Enforcement Agency has increased steadily from 3.5 percent in 1994 up to 12.3 percent in 2012.
Meanwhile, average risk of developing cannabis use disorder increased by about 40 percent with every 1 percentage point increase in national pot potency, they found.
"It definitely is a signal that potency increases the addictive potential of cannabis," said senior author Brian Hicks, an associate professor with the University of Michigan Addiction Center. And there are no laws regarding potency, he added.
States with legal marijuana might want to hone their laws to make THC levels in pot products clearer for consumers, just as alcohol levels are displayed on liquor bottles, the researchers said. Currently, 10 states and Washington, D.C., allow recreational marijuana use and many more states allow medical use.
"We really need to look at the idea of regulating potency levels, and examining how potency relates to things you'd care about, like ability to drive and addiction potential," Hicks said.
"Levels have really gone up the last five years," particularly in pot products such as edibles and concentrates, he added.
NORML, an advocacy group for marijuana legalization, took issue with the findings, however.
Some prior studies have suggested that cannabis use disorder rates in the United States are actually declining, not rising, NORML deputy director Paul Armentano said. And he added that "cannabis lacks the dependence liability associated with many other controlled substances, including those that are legal."
In the new study, Hicks and his colleagues compared federal statistics on pot potency with data gathered on participants in the Michigan Longitudinal Study. That's an ongoing research project focused on families at increased risk for substance abuse disorders.
They specifically looked for symptoms related to cannabis use disorder, a condition recognized by the American Psychiatric Association. These symptoms might include an inability to cut down or control cannabis use, failing to meet obligations due to pot use, or using the drug even in physically hazardous situations, Hicks said.
The researchers found that regular pot users who first tried marijuana when national average THC levels held at 4.9 percent had almost twice the increased risk of developing symptoms of cannabis use disorder within a year.
But those who started regularly using pot when national average THC levels were 12.3 percent had a 4.8 times higher risk of cannabis use disorder.
The study is "one of the first to show increased risk of progression to cannabis use disorder with higher potency marijuana," said Dr. Scott Krakower, assistant unit chief of psychiatry with Zucker Hillside Hospital in Glen Oaks, N.Y.
It's no surprise that the more potent pot is, the higher the risk that it can lead to problematic use, said Dr. Harshal Kirane, director of addiction services at Staten Island University Hospital in New York City. Both New York experts were not involved with the study.
"A consistent principle of the biology underlying addictive behavior is that the potency of a substance usually correlates with its addictive potential," Kirane said.
However, at this time the United States doesn't have the knowledge to accurately set standard THC levels, he continued.
"The actual evidence base for what constitutes a reasonably safe level of THC remains unknown," Kirane said. "In the same vigor by which we're pushing these products, we need to fund and support research efforts to adequately characterize the health risks involved in THC use and consumption."
Hicks agreed that future research needs to focus on practical matters relating to THC levels in pot, much as alcohol research helped policy makers eventually come up with legal blood alcohol levels that indicate inebriation.
Right now, Kirane said the question, "how do we come to terms with a healthy relationship to cannabis in the same way we continue to struggle with having a healthy relationship with alcohol?" is the bigger conversation for society.
"It's a legal substance, but it has the potential for major health consequences," Kirane said.
NORML's Armentano said pot's risk for dependence needs to be kept in perspective.
"According to the National Academy of Sciences' Institute of Medicine, cannabis' risk of causing dependence is far lower than that of alcohol, opiates, or tobacco," he said.
"At worst, cannabis' dependence impacts about1 in 10 users -- a figure that is on par with anxiolytics [anti-anxiety drugs] such as Valium or Xanax, and is just slightly more prevalent than the percentage of Americans dependent upon caffeine," Armentano said.
The new study appears in the Dec. 17 issue of Drug and Alcohol Dependence.