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Medical Study Shows Early Marijuana Use is Linked to Increased Risk of Abuse or Dependence A recent study indicates that people who use marijuana before the age of 17 may be at a higher risk for drug abuse or dependence. The study was conducted by Dr. Michael Lynskey with colleagues at the Washington University School of Medicine in St. Louis, Missouri; the Queensland Institute of Medical Research in Brisbane, Australia; and the University of Missouri in Columbia.

The study was conducted by interviewing 311 pairs of same-sex twins (average age over 30). One of the twins admitted to having smoked marijuana before the age of 17 and other twin had either never smoked the drug or did so for the first time at age 17 or older. The study is significant in that the twin pairs had the same biological characteristics, and had common social, economic and family environments. They grew up in the same households and attended the same schools. Thus, the outside influences were the same for both twins.

The researchers found that the twin who smoked marijuana before the age 17 was more than twice as likely as their sibling to use opioids, three times as likely to use cocaine or other stimulants, and nearly four times as likely to use hallucinogens. The twin who smoked marijuana before age 17 also was more likely to have reported abuse or dependence on either alcohol or an illegal drug. However, the majority (52%) did not go on to develop abuse or dependence.

"While these study findings indicate that early marijuana use is associated with increased risk of progression to other illicit drug use and possibly to drug abuse or dependence, it is not possible to draw strong causal conclusions solely on the basis of these associations," Dr. Lynsky stated. Additional research will be needed to determine the causes of the association.

"Given that early initiation of marijuana smoking appears to be associated with increased risks," says Dr. Lynskey, "there is a need for greater physician awareness of those risks. Focused interventions are also needed to prevent escalation to use of other drugs among young people identified as being at risk."

Source: NIDA Notes, Volume 18, Number 4, published by the National Institute on Drug Abuse, National Institutes of Health, U.S. Department of Health and Human Services.

Cognitive Deficits in Marijuana Smokers Persist After Use Stops

A study funded by the National Institute on Drug Abuse (U.S. Department of Health and Human Services, National Institutes of Health) has found that cognitive impairment developed as a result of smoking marijuana can last for up to 28 days after the person last smoked the drug. The more a person smoked, the more profound the impairment, and persons with lower IQs had more profound impairment than persons with higher IQs who had smoked more of the drug.

Twenty-two individuals were tested. Each individual had smoked marijuana for an average of 4.8 years. They were grouped as light, medium or heavy smokers, with heavy abusers smoking an average of 94 joints per week and light abusers smoking an average of 11 joints per week. The heavy smokers scored worse on 24 of the 35 neurocognitive tests, even after not having smoked for 28 days. The tests in which the heavy abusers had comparative deficits included verbal and visual memory, visual perception, executive functioning, psychomotor speed, and manual dexterity. Dr. Karen Bolla, one of the researchers, stated "[t]he more marijuana people used, the worse they performed on the tests, especially those for memory."

"We know a lot about the acute effects of marijuana use, but researchers are just now beginning to look at the long-term effects," says Dr. Jag Khalsa of NIDA. "This study demonstrates that marijuana smoking has chronic, dose-related effects on cognitive impairments up to 28 days after last use. But how long do these effects persist beyond that point? That's something we have to examine."

The findings from this study were consistent with an earlier study conducted by Dr. Harrison Pope, Jr. at Harvard University McLean Hospital, which found that memory and learning problems caused by heavy marijuana smoking last at least a week after use of the drug.

Source: NIDA Notes, Volume 18, Number 5, published by the National Institute on Drug Abuse, National Institutes of Health, U.S. Department of Health and Human Services.

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