The study found that frequent marijuana users performed worse than non-users on tests of cognitive abilities, including divided attention (ability to pay attention to more than one stimulus at a time) and verbal fluency (number of words generated within a time limit). Those who had used marijuana for 10 years or more had more problems with their thinking abilities than those who had used marijuana for five to 10 years. All of the marijuana users were heavy users, which was defined as smoking four or more joints per week.
"We found that the longer people used marijuana, the more deterioration they had in these cognitive abilities, especially in the ability to learn and remember new information," said study author Lambros Messinis, PhD, of the Department of Neurology of the University Hospital of Patras in Patras, Greece. "In several areas, their abilities were significant enough to be considered impaired, with more impairment in the longer-term users than the shorter-term users."
The study involved people ages 17 to 49 taking part in a drug abuse treatment program in Athens, Greece. There were 20 long-term users, 20 shorter-term users and 24 control subjects who had used marijuana at least once in their lives but not more than 20 times and not in the past two years. Those who had used any other class of drugs, such as cocaine or stimulants, during the past year or for more than three months throughout their lives were not included in the study. Before the tests were performed, all participants had to abstain from marijuana for at least 24 hours.
The marijuana users performed worse in several cognitive domains, including delayed recall, recognition and executive functions of the brain. For example, on a test measuring the ability to make decisions, long-term users had 70 percent impaired performance, compared to 55 percent impaired performance for shorter-term users and 8 percent impaired performance for non-users. In a test where participants needed to remember a list of words that had been read to them earlier, the non-users remembered an average of 12 out of 15 words, the shorter-term users remembered an average of nine words and the long-term users remembered an average of seven words.
The American Academy of Neurology, an association of more than 19,000 neurologists and neuroscience professionals, is dedicated to improving patient care through education and research. A neurologist is a doctor with specialized training in diagnosing, treating and managing disorders of the brain and nervous system such as stroke, Alzheimer's disease, epilepsy, Parkinson's disease, autism and multiple sclerosis.
For more information about the American Academy of Neurology, visit www.aan.com.
Last reviewed: By John M. Grohol, Psy.D. on 21 Feb 2009
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