A new comparative study found that people using large amounts of marijuana for a long time perform worse at standard memory and decision-making tests. The study hasn't yet determined whether the changes are irreversible. Lambros Messinis of the Department of Neurology at the University Hospital of Patras in Greece has tested 64 people, age 17 to 49, falling into three groups: 20 who had smoked four or more joints per week for more than 10 years (for an average of 15 years); 20 who'd been smoking for 5 to 10 years (for an average of 7 years); and 24 controls who had smoked at least once but not more than 20 times and not at all in the past two years. The subjects were not allowed to smoke at least 24 hors before the test, to make sure they were not still under the influence. "It wasn't while they were high," he said.The results were rather stark: "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 Messinis. "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." For example, long-term users performed worse in memory tests such as the one testing the ability to remember a list of words. Asked to recall lists of 15 words that they had seen earlier, the long-term users averaged 7, compared with 9 recalled by short-term users and 12 by controls. On a decision-making test, the performance of long-term users was 70 percent impaired, compared to 55 percent impaired in cases of shorter-term users and 8 percent impaired performance for non-users. Frequent marijuana users also performed worse than medium users and non-users on tests of cognitive abilities, such as divided attention (the "multi-tasking" ability to pay attention to more than one stimulus at a time) and verbal fluency (number of words generated within a time limit). Medium users performed worse than non-users."It definitely fogs your brain," says Messinis. "Importantly, we don't know if it's reversible." To find this, he and his colleague plan to re-evaluate the same subjects after an abstinence period of at least a month.Other studies have shown that marijuana has lasting memory impairment effects in case of people who have started smoking marijuana before age 17, when the brain is still forming. This troubles scientists and doctors because, according to the National Institute on Drug Abuse, there are around 3.1 million Americans age 12 and older who use marijuana daily or almost daily. In 2004, 5.6 percent of 12th graders reported daily use of marijuana.