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it is widely believed that cannabis is not an addictive drug, but a new study challenges this belief. Among cannabis-using adolescents undergoing treatment for substance abuse, researchers from Massachusetts General Hospital found that 40% displayed withdrawal symptoms - a hallmark of drug addiction.

This study comes as New York was recently made the 23rd state to legalize medical marijuana. In addition, the city of Berkeley, CA, recently passed legislation that requires marijuana dispensaries to offer free medical marijuana to patients with low incomes.

In 2012, Washington and Colorado became the first states to legalize marijuana for recreational use among adults, and Alaska and Oregon are expected to vote on the issue later this year.

"Our results are timely given the changing attitudes and perceptions of risk related to cannabis use in the US," says senior study author John Kelly, PhD, of the Center for Addiction Medicine in the Department of Psychiatry at Massachusetts General Hospital.

"As more people are able to obtain and consume cannabis legally for medical and, in some states, recreational use, people are less likely to perceive it as addictive or harmful. But research shows that cannabis use can have significant consequences, and we know that among adolescents it is second only to alcohol in rates of misuse."

Kelly says many studies have investigated cannabis withdrawal among adolescents being treated for substance use disorders, but he notes that very few have followed-up patients for longer than 30 days.

In addition, he says the majority of these studies have not looked at the association between withdrawal symptoms and the frequency of cannabis use and whether patients have other psychiatric symptoms.

Kelly and his colleagues wanted to address these points in this latest study, results of which are published in the Journal of Addiction Medicine.

Adolescents with cannabis withdrawal symptoms 'more likely to be addicted'

The team assessed 127 adolescents aged 14-19 who were being treated at an outpatient clinic for substance use disorders. Of these, 90 reported cannabis as their drug of choice.

Fast facts about cannabis use in the US

  • Cannabis, or marijuana, is the most common illicit drug used in the US
  • In 2010, cannabis was used by 76.8% of current illicit drug users, defined as using the drug within the past 30 days
  • A 2009 survey found that cannabis was a contributing factor in more than 376,000 emergency department visits in the US.

At study baseline, participants were asked about their withdrawal symptoms, whether they had been diagnosed with any psychiatric disorders or symptoms, whether they thought they had a problem with drug use, and any consequences in their lives they believed were a result of drug use. These assessments were completed again 3, 6 and 12 months later.

The participants were then divided into one of two groups dependent on their answers: those who reported withdrawal symptoms from cannabis use - including sleeping problems, anxiety and depression - and those who had no withdrawal symptoms.

Kelly and colleagues found that of the 90 participants who frequently used cannabis, 76 (86%) met the criteria for cannabis dependence. These criteria included increased tolerance or use of cannabis, failed efforts to reduce or stop using the drug, and continued use despite the drug making any psychiatric conditions worse.

The team found that adolescents who reported withdrawal symptoms, however, were more likely to meet the criteria for cannabis dependence. Results revealed that 36 of the 90 participants (40%) reported withdrawal symptoms and all of these met cannabis addiction criteria.

These subjects were also more likely to have mood disorders and report severe cannabis use and negative life consequences as a result of their drug use, such as financial and relationship problems and absence from work or school.

Recognition of drug problem linked to reduction in cannabis use

The researchers note that although withdrawal symptoms appear to be a sign of cannabis addiction, they did not appear to influence participants' ability to reduce cannabis use throughout the study period.

However, adolescents with withdrawal symptoms who said they recognized they had a cannabis use problem were better able to reduce their use of the drugs over the 12-month period.

Those with withdrawal symptoms who did not recognize they had a drug problem showed a slight reduction in cannabis use in the first 3 months, but their drug use increased thereafter. This pattern was also seen among cannabis users who did not have withdrawal symptoms.

The team believes that individuals who have withdrawal symptoms but who do not recognize they have a drug problem may not associate their symptoms with cannabis withdrawal.

"Those who do acknowledge a substance-use problem may correctly attribute those symptoms to cannabis withdrawal, giving them even more motivation to change their substance use behavior," says study co-author Claire Greene, a doctoral candidate at Johns Hopkins Bloomberg School of Public Health in Baltimore, MD.

Commenting on their study results, Kelly says:

"The importance of understanding the addictiveness, risks and harms associated with cannabis use is a major theme of this study's findings. Recognizing those risks is known to reduce the likelihood that someone will start to use drugs, and better understanding of the role of substances in the problems experienced by patients may help them cut down on future use."

However, he notes that people in the US tend to focus on reducing the risks associated with cannabis use rather than recognizing its addictiveness - something he believes is an issue.

"Further research is needed determine the impact of these changing public attitudes and investigate the benefits of programs that reduce these misconceptions, which could allow us to predict whether increased education and awareness could help reduce the onset of, and harm caused by, cannabis use disorders."

Medical News Today recently reported on a study claiming states that legalize marijuana have fewer deaths from prescription drug overdose.

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