March 27, 2011Dear Sir,Those who accept or promote any legalisation of marijuana should take heed of recent published scientific studies which have found that exposure to marijuana has far reaching and serious effects on the brain, especially memory functions, judgment and mental health. The most recent data comes from a peer-reviewed study published in the prestigious British Medical Journal in which researchers followed 1,923 adolescents and young adults aged 14 to 24 for ten years and were able to prove that marijuana use was linked to the development of psychotic disorders, hallucinations and delusions, and was thus considered to be a contributory cause of schizophrenia, essentially doubling the risk.Another recent study from Harvard Medical School was presented at the last meeting of the Society for Neurosciences and evaluated 35 chronic marijuana smokers and compared them to 29 healthy people. Twenty of the smokers started before age 16, a further 15 started after age 16, and the average age at the time of evaluation was 22. The subjects were given a standard test of brain executive function, which involves planning, abstract thinking and moderating one’s social behaviour, the Wisconsin Card Sort Task. See en.Wikipedia.org/wiki/Winconsin_card_sort for an explanation of the test. The non-smokers performed better than the smokers, and the early-onset smokers did worse than the late-onset smokers, in some areas making twice as many errors. These executive functions are performed in the last area of the brain to develop in adolescents, the prefrontal cortex, and the marijuana-induced damage appears to be irreversible.I have reviewed some 38 other scientific studies which have appeared in the medical literature over the last two years which generally support the above findings, without contradictions. For brevity’s sake I will not review these studies, although the references are available. Other studies have shown that there are also deleterious effects of marijuana smoking on lung functions similar in many ways to those of cigarette smoke. Of course, these brain and lung effects presumably then have many and widespread additional ramifications for society, leading to increased rates of mental health and lung diseases, hospitalisations and incarcerations. Therefore, the use of marijuana, especially in younger age groups, should not be encouraged by any legalisation, and should continue to be discouraged by specific programmes. Regarding any comparison with alcohol, this chemical has equally bad individual health implications and societal ramifications, but that genie is already out of the bottle. We should not repeat this mistake.KEITH H CHIAPPA MDNeurologis