The original post in its native form was far too long, so I have decided to break it up into seven different posts, in addition to this post. The separate sections are listed below. The original post, what is left of it, is here. For an examination of the evidence of whether or not cannabis causes actual structural damage to brain cells, axons or dendrites, see here. For an analysis of neuropsychological batteries of cannabis users to determine whether or not they suffer brain damage, see here. For an analysis of EEG testing of cannabis users to discover evidence of brain damage, see here. For an analysis of studies looking at cerebral blood flow in cannabis users, see here. For an admittedly impressionistic analysis of whether or not cannabis causes schizotypal symptoms in users, see here. For a summary comparing the effects of cannabis on the brain compared to other drugs, see here. Original monkey and later rat studies indicating structural brain damage have generally not panned out when conducted in humans, but a recent study from 2008 found damage to the hippocampus and amygdala. The hippocampus findings are contradicted by three earlier studies finding no damage, and the finding on the amygdala was contradicted by an earlier study. Another study found damage to the corpus callosum in early adolescent users. And another found damaged axons and brain cells in the prefrontal cortex. These findings have not yet been replicated. Yet another study found damage to two areas but improvements in four other areas of the brain. The consequences of this are not known. At the moment, whether or not cannabis causes structural damage or even improvements to the limbic system, corpus callosum, prefrontal cortex or other areas of the brain is somewhat up in the air. Neuropsychological studies of long-term users have been somewhat contradictory, but in general have not found significant brain damage, although they did find a “very small effect” on learning and memory. Apparently they did not think it was significant enough to be called brain damage. New studies show that chronic long-term users perform worse than controls in memory, learning and recall, and the effect worsens as use progresses. These effects last up to one week after the last use. Therefore, daily marijuana users are always going to be somewhat impaired in these areas. Some EEG studies found some interesting changes in theta waves in users who had used heavily and daily for 15-30 years. The significance of these findings is not yet known; one suggestion that is that the increased theta may be indicative of organic damage. But here again we do not have any clinical correlates of the organic theta change either. Even this study did not find permanent effects from sporadic or occasional use of marijuana. Another found problems with screening out external stimuli after five years of use, but it is possible that two attempts to replicate that study may have failed. A SPECT studies out of Britain showed low CBF in three heavy cannabis users. Another found that low CBF did not clear up in adolescent users even after one month. However, they have been contradicted by two other studies, one showing that the CBF deficits clear up after three months, and another showing increases in CBF as opposed to decreases. A new study using DTI found damage to the left side of the arcuate fasiculus in adolescent heavy cannabis users. This is one of the most disturbing findings to date, and adds weight to evidence that cannabis should not be used by adolescents, since the arcuate fasiculus is still developing in early adolescence. Nevertheless, no clinical significance has yet been attached to this finding. But another DTI study looking at the entire brain found no evidence of generalized damage, and if anything, found that teenage cannabis users have less brain atrophy and more brain cells than non-users. We would expect a clinical correlate of this to be more intelligent teenage potheads, but no one is suggesting that. Unless clinical correlates can be discovered, all findings of damage or enhancement of the brain via marijuana should be viewed with a critical eye. So the evidence on permanent brain damage from long-term heavy marijuana use is rather contradictory and is still somewhat up in the air. It looks like cannabis can be used for up to five years, or possibly up to 15 years, even on a daily basis, without any permanent harm to the brain. Beyond that, there may effects, but they appear to be more subtle than profound.
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