Don't say you haven't been warned. New research\u00a0has found\u00a0that even casual marijuana use causes critical changes in the brains of young people from which they may never recover and should prompt President Barack Obama to rethink\u00a0a view he expressed\u00a0in\u00a0January\u00a0about\u00a0marijuana\u00a0being no more dangerous than alcohol, one scientist said. The first-of-its-kind study\u00a0on recreational marijuana, published April 16 in the Journal of Neuroscience,\u00a0was a collaboration between Northwestern University's medical school, Massachusetts General Hospital and Harvard Medical School. Previous studies have revealed brain changes in chronic marijuana users, but this new research shows such abnormalities in young, casual smokers. Regions of the brain linked to motivation and emotion are hijacked by even once-a-week use of marijuana, said co-senior study author Anne Blood, director of the Mood and Motor Control Laboratory at Massachusetts General Hospital and assistant professor of psychiatry at Harvard Medical School. Even with sustained periods of abstinence, it's unlikely the brain can ever go back to exactly how it was,\u00a0Blood said, likening the problem to that\u00a0faced by stroke victims. They may recover some function, but once the brain is changed, "it's very hard to go back," she said. "What I don't want someone to say is that, 'No one told me I'd turn into this depressed couch potato,' " a few years down the road, Blood told Promises Behavioral Health. "This study is not a judgment on marijuana use, but if marijuana is changing the brain from purely recreational use, people have a right to know that. Until this study, there was a real lack of information about casual use." The\u00a0researchers looked at the brains of 40 people aged 18 to 25, half of whom smoked marijuana and\u00a0half who didn't,\u00a0and found that the size, density and shape of the amygdala and the nucleus accumbens -- key regions in the brain\u00a0for motivation and\u00a0emotion\u00a0-- were abnormally altered in those who did smoke marijuana.\u00a0The nucleus accumbens was enlarged, and its alteration in size, shape and density was directly related to how many joints an individual smoked. The changes in brain structures indicate the marijuana users\u2019 brains are adapting to low-level exposure to marijuana, the researchers\u00a0said. Scientists have known for some time that chronic,\u00a0long-term use of marijuana has lasting effects on the brain. A recent study of\u00a0pot smokers\u00a0who began using in adolescence revealed substantially reduced connectivity among brain areas responsible for learning and memory,\u00a0according to the National Institute on\u00a0Drug Abuse. \u00a0And a large long-term study in New Zealand showed that people who began smoking marijuana heavily in their teens lost an average of 8 points in IQ between age 13 and age 38. Importantly, the lost cognitive abilities were not fully restored in those who quit smoking marijuana as adults. How Do Brain Changes Affect Students? Marijuana is the most commonly used illicit drug in the U.S. with an estimated 15.2 million users, the study reports, based on the National Survey on Drug Use and Health in 2008. The drug's use is increasing among adolescents and young adults, partially due to\u00a0society's changing beliefs about cannabis use and its legal status. An\u00a0NBC\/Wall Street Journal poll\u00a0in March\u00a0found that only 8% of adults think that marijuana is the most harmful substance to a one\u2019s overall health when\u00a0compared with\u00a0 tobacco, alcohol and sugar. In contrast, 49%\u00a0rated tobacco as the most harmful on the list, while 24% mentioned\u00a0alcohol. Because teen brains are only about 80% developed and don't fully mature until about age 25,\u00a0adolescents\u00a0are at particular risk for suffering adverse effects from drug use. But quantifying those effects\u00a0is sometimes\u00a0difficult. A teen who smokes pot on the weekends may be pulling decent grades, but it's\u00a0hard to know how much better that kid's grades would have been if he hadn't used, Blood said. "It's hard to know what life would have been like if they hadn't smoked," she said. "So much about grades comes from motivation and emotion," she said. "Does a subject 'speak' to you? Drugs can narrow the range of things that interest students. Maybe they\u00a0played an instrument or\u00a0were on a sports\u00a0team in high school and then\u00a0they drop those in college.\u00a0\u00a0If you bust your motivation circuitry, that's what can happen. There's no question that the way the brain is treated affects motivation." Basic Life Functioning Blood calls motivation a critical human function that "we sort of take for granted." "These regions of the brain controlling motivation and emotion affect life so broadly," Blood said. "They are very involved in basic life functioning. When you wake up in the morning, are you even motivated to get out of bed? Teens' parents can pull them out of that bed, but when they're on their own, are\u00a0[the marijuana users]\u00a0going to be able to make something happen in their life? Be able to take responsibility?" The new\u00a0research fits with animal studies that show when rats are given tetrahydrocannabinol (THC), the principal psychoactive ingredient in marijuana, their brains rewire and form many new connections, the university said in a news release.\u00a0 In animals, these new connections indicate the brain is adapting to the unnatural level of reward and stimulation from marijuana. These connections make other natural rewards less satisfying. "Everyone wants to derive pleasure from life," Blood said. "The last thing you want to do is limit your ability to find it." Blood asserts that more studies are needed to\u00a0learn the long-term effects of casual marijuana use. But she stresses the importance of adolescents staying away from drugs that alter the structure of their brains, which\u00a0remain a work in progress until they are in their mid-20s. "People just don't think about the thing that's making them think,"\u00a0Blood said. As for President Obama's\u00a0comment? "I'm sure\u00a0the president said that in good faith," Blood said. "He was speaking for what the public believes." Previous\u00a0research has found that alcohol alters the brain, said study co-author Hans Breiter, a psychiatrist and mathematician at the Northwestern's Feinberg School of Medicine in Chicago. But while researchers do not know exactly how the mental rewiring seen in\u00a0marijuana users affects their lives, the new study shows it physically\u00a0changes the brain in ways that differ from drinking,\u00a0Breiter said.