S.18 Cannabis use in adolescents: effects on brain and behaviour S203 and mental abilities. This study is comparing cannabis users and non-users who are 14 to 19 years old and is examining cognitive domains that include working memory, associative memory, and the influence of reward. Many individuals begin using cannabis during this age range. It is possible that individuals who begin using cannabis at earlier ages may experience greater adverse effects than those who begin at later ages, especially considering that brain development continues throughout this age range. In the United States this international collaborative study derives from our ongoing and past positron emission tomography (PET) and structural MRI studies of chronic cannabis users. Our previous studies showed alterations of regional brain blood flow in adult chronic cannabis users, relative to non-users, particularly in the cerebellum, prefrontal cortex, and hippocampus, during episodic memory [1] and selective attention tests. Chronic cannabis users also showed substantially lower blood flow than controls in a large region of posterior cerebellum while lying quietly [2]; but did not show any noteworthy structural changes in the brain [3]. Our ongoing PET and structural MRI studies are expanding this research by examining the influence of age of first use of cannabis and duration of use on cognition-related regional brain flood flow, brain structure, and mental abilities. References [1] Block RI, O’Leary DS, Hichwa RD, Augustinack JC, Boles Ponto LL, Ghoneim MM, Arndt S, Hurtig RR, Watkins GL, Hall JA, Nathan PE, Andreasen NC, 2002, Effects of frequent marijuana use on memory- related regional cerebral blood flow. Pharmacol Biochem Behav 72, 237–250. [2] Block RI, O’Leary DS, Hichwa RD, Augustinack JC, Boles Ponto LL, Ghoneim MM, Arndt S, Ehrhardt JC, Hurtig RR, Watkins GL, Hall JA, Nathan PE, Andreasen NC, 2000, Cerebellar hypoactivity in frequent marijuana users. Neuroreport 11, 749–753. [3] Block RI, O’Leary DS, Ehrhardt JC, Augustinack JC, Ghoneim MM, Arndt S, Hall JA, 2000, Effects of frequent marijuana use on brain tissue volume and composition. Neuroreport 11, 491–496. S.18.03 European epidemiology of cannabis use in adolescents: determinants and risk factors M. Choquet ° , C. Huas, N. Chau, C. Hassler. Inserm, 669, Paris, France Cannabis concerns 20% of the Europeans 15 to 65 year olds (OEDT, 2006). In 2003, according to the ESPAD study (Hibell et al, 2004), (a) 21% of 16 years olds experienced cannabis during lifetime, 9% during last 30 days; (b) 35% perceived cannabis as “easily” available, 14% as easily available in school; (c) 32% perceived experimental use at “great risk”, 70% regular use. But these findings hide an important heterogeneity between countries, not only on prevalence rates (e.g. lifetime prevalence ranged from 3% in Romania to 44% in Czech Republic, last 30-days prevalence from 1% in Sweden to 22% in France), but also on risk factors (e.g. correlation between parental education and cannabis ranges from being significantly negative to being significantly positive). In France (Choquet et al, 2004), where cannabis use is one of the highest of Europe, results suggests that (a) Boys, 17−18 years olds and those from high educated parents are more at risk than girls, 12−13 years olds and children from low educated parents; (b) not only regular users but also occasional users (1−5 times/last 30 days) are at risk of suicide attempt (OR = 4.2), regular school absenteeism (OR = 4.8), run away (OR = 4.2) and violent behavior (OR = 12.6); (c) lack of parental control increases the risk of consumption (OR between 3.5 and 6.4 according to gender and family structure) while emotional support is not as relevant (OR between 0.8 and 4.5). Prevention has to taken into account these cultural differences across Europe. References [1] OEDT, 2006, Annual report 2006: the state of the drugs problem in Europe. Lisbonne. [2] Hibell B, Andersson B, Bjarnasson T, Ahlstr¨ om S, Balakireva O, Kokkevi A, Morgan M, 2004, The ESPAD report 2003. Alcohol and other drug use among students in 35 European countries. Stockholm: CAN. [3] Choquet M, Beck F, Hassler C, Spilka S, Morin D, Legleye S, 2004, Les substances psychoactives chez les coll´ egiens et lyc´ eens: consommations en 2003 et ´ evolutions depuis dix ans. OFDT tendances, Nº. 35. S.18.04 Cannabis use in adolescents: effects on neurocognitive brain function investigated with fMRI G. Jager 1 ° , R.I. Block 2 , M. Luijten 1 , J.M. Zuccarelli 2 , M.M. Becker 2 , N.F. Ramsey 1 . 1 University Medical Center Utrecht, Department of Neurosurgery, Utrecht, The Netherlands; 2 University of Iowa, Department of Anesthesiology, Iowa City, USA Cannabis use continues to be a major issue in both the US and the European Union. In the Netherlands, about 7% of 15−16 year old adolescents use more than 6 cannabis cigarettes per month, while 9% do so in the US [1]. Because of the widespread use pattern, potential adverse effects of chronic cannabis use on human brain function, brain structure and cognition in adolescents are of serious concern, especially in the context of ongoing brain maturation during adolescence [2]. However, very little is known about the long-term effects of chronic cannabis use on adolescent brain function or structure, or the relationship of these effects to age of onset or duration of use. In 2006 a Dutch-US international collaborative research project has started to investigate the sustained effects of chronic cannabis use on cognition and brain function during adolescent development. This project extensively examines 24 male cannabis users (at least one week abstinent) between 14 and 19 years of age, and 24 male cannabis-naive controls, matched for age, IQ, alcohol and tobacco use, on cognitive performance and brain function using functional MRI. Half of the sample was recruited and examined in the US and the other half in The Netherlands, using the same procedures and fMRI scanning paradigms on both locations. Afterwards, data were pooled. We examined brain function in various domains: working memory, associative memory and reward. References [1] The Netherlands National Drug Monitor Annual report 2005, Trimbos Institute. www.trimbos.nl. [2] Viveros MP, Llorente R, Moreno E, Marco EM, 2005, Behavioural and neuroendocrine effects of cannabinoids in critical developmental periods. Behav Pharmacol 16(5−6), 353–362.