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NIDA. (1999, April 20). Chronic Marijuana Users Become Aggressive During Withdrawal. Retrieved from

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April 20, 1999

People who have smoked marijuana daily for many years display more aggressive behavior when they stop smoking the drug, according to a new study by researchers at Harvard Medical School. The study, funded by the National Institute on Drug Abuse (NIDA), National Institutes of Health, is further evidence that a withdrawal syndrome is associated with abstinence from long-term marijuana use, and suggests that aggressive behavior is part of this syndrome.

Human and animal studies conducted since the early 1970s have suggested the existence of a marijuana withdrawal syndrome, characterized by insomnia, restlessness, loss of appetite, and irritability. "This syndrome - although less dramatic than the withdrawal syndrome associated with alcohol, opiate, or cocaine withdrawal - may contribute to relapse among those dependent on marijuana," says NIDA Director Dr. Alan I. Leshner. "People addicted to marijuana may continue to use the drug at least partly to prevent the onset of withdrawal symptoms. Identifying the exact nature of this syndrome is crucial to developing treatment strategies for those attempting to stop their marijuana use."

The Harvard study, using a computer test of aggressive behavior, compared 17 long-term heavy users of marijuana with 20 people who were infrequent or former smokers. All of the volunteers were abstaining from the use of marijuana and all other drugs of abuse during the study. To avoid any suggestive influences, the investigators did not reveal to the volunteers that their aggressive behavior was being studied. Instead, the volunteers were told that they were participating in a test designed to measure movements and a variety of physiological characteristics, such as pulse rate and body temperature. They were also told that another volunteer of the same sex was sitting in another experimental chamber and would be participating in the test with them.

Facing each volunteer in the experimental chamber was a computer monitor and a response panel. On the panel were two buttons, labeled A and B. Pressing A 100 times allowed the volunteer to gain 1 point, which was worth 50 cents. Pressing B 10 times subtracted 1 point from the other participant, who, in actuality, was not a person but a computer. Three 1-second beeps signaled that the "opponent" was subtracting 1 point from the volunteer. The researchers labeled the pressing of B as an aggressive response, and the point subtractions from the volunteer as provocations.

The volunteers participated in one of these sessions on each of five different occasions: at the beginning of the study, and at days 1, 3, 7, and 28 of abstinence. Results showed that the current marijuana users became significantly more aggressive, as indicated by the number of times they pressed B, on days 3 and 7 of withdrawal, as compared to the infrequent or former marijuana users.

"Most of the studies that have been published on marijuana withdrawal symptoms in people have relied on self-report," says Dr. Elena Kouri, lead author of the paper. "In these studies, long-term marijuana users report that they feel irritable when they are abstaining from marijuana use, but these studies generally do not involve measurements of aggressive behavior to verify these self-reports. In our study, we demonstrated that long-term marijuana users do, indeed, exhibit more aggressive behavior during the first week of abstinence, and that this aggressive behavior can be measured."

Marijuana is the most widely used illicit drug in the United States. More than 11 million people have smoked marijuana within the past month, according to the 1997 National Household Survey on Drug Abuse.

Long-term marijuana use can lead to addiction in some people. These marijuana-addicted individuals use the drug compulsively, and this use often interferes with family, school, work, and recreational activities. Individuals with cannabis dependence may also persist in using the drug despite knowing that it causes them physical problems, such as a chronic cough related to smoking, or psychological problems, such as excessive sedation due to high doses.

"Although it is difficult to be certain of the exact prevalence of cannabis addiction in the United States, I can tell you anecdotally that we had no difficulty recruiting dozens of people between the ages of 30 and 55 who have smoked marijuana at least 5,000 times," says Dr. Harrison Pope, Jr., principal investigator of the study. "A simple ad in the paper generated hundreds of phone calls from such people."

This study on changes in aggressive behavior during withdrawal from long-term marijuana use appears in the April issue of Psychopharmacology.