So far this has been a two-part discourse. The first part was my elucidating of a study on marijuana usage, and the second part I’ll post in a “reply-post” or two. It contains his reply in full, and in order, with my responses interspersed.
A study called “The Toxicity of Cannabis”, available here (click “Get PDF” at the bottom), published in a journal of chemistry and biodiversity, concludes (page 20 of the pdf, with my bolding): The recently observed increase in cannabis use, particularly among younger people, poses potential harm to individuals and society at large, and must clearly be attended to by society. The extent of the risks associated with cannabis use depend on several factors. According to the above review, young age among users, regular and heavy use, and the smoking of cannabis cigarettes are factors most likely to increase the negative effects of cannabis use. The greatest concerns should be associated with minimizing cannabis use by young adolescents. Potentially negative long-term effects of cannabis use, including dependency, induction of psychosis, effects on cognition and academic achievement are more severe in young people than in adults.Some long-term effects are summarized this way in the abstract: Effects on cognitive abilities seem to be reversible after abstinence, except possibly in very heavy users.A very relevant paragraph on page 14 of the pdf reads: Among these studies is the one by Pope et al. who compared three groups of individuals aged 30 to 55 years with regard to their cognitive abilities following cessation of cannabis use: 63 current heavy users of cannabis, 45 former heavy users, and 72 nonusers . Some cognitive deficits were detectable at least seven days after discontinuation of heavy cannabis use. By day 28, however, there were virtually no significant differences among the groups on any of the test results, suggesting that cannabis effects on cognition in heavy users appear to be reversible and related to recent cannabis exposure. However, another investigation suggests that very heavy use of cannabis may be associated with irreversible decline in cognitive performance .Footnotes: H. G. Pope, A. J. Gruber, D. Yurgelun-Todd, Curr. Psychiat. Rep. 2001, 3, 507.
 K. I. Bolla, K. Brown, D. Eldreth, K. Tate, J. L. Cadet, Neurology 2002, 59, 1337.Cognitive deficits are apparent at least after seven days! That’s significant – and that’s not just for heavy users. The same study states (see below) that long-term effects in regular/moderate users can be reversed if subjects stop using the drug a week prior to testing – which makes sense, because if they stop using it there’s no reason to expect they won’t recover. Nevertheless, that shows that moderate users AND heavy users BOTH test poorly in tests of cognitive function, unless they quit the drug; and it takes heavy users significantly longer to recover (up to 28 days). Both durations of impairment are significant to any average person in society, unless he stays at home for a week (or 28 days) and doesn’t use the drug during that time.
Text of that section (page 14 of the pdf, with my bolding): Participants were divided according to the number of joints smoked per week into light users (mean: 11 joints), medium users (mean: 42 joints), and heavy users (mean: 94 joints). Very heavy users performed significantly worse on 5 of the 35 tests compared to light users even after 28 days of controlled abstinence. Memory, executive functioning, psychomotor speed, and manual dexterity were affected. In a more recent study, no differences in the performance of moderate regular cannabis users, who were abstinent from the drug for one week, and non-users in tasks on working memory and selective attention were observed .Footnote:  G. Jager, R. S. Kahn, W. Van Den Brink, J. M. Van Ree, N. F. Ramsey, Psychopharmacology 2006, 185, 358. I’d love to see you refute this information.