Repost from the old site.
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 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.
For a summary of the findings of cannabis and brain damage, see here.
One of the ways of determining brain damage from the use of drugs is to subject users to neuropsychological batteries. A neuropsychological battery is a test that is specifically designed to pick up brain damage. This post will deal with the results of neuropsychological batteries of cannabis users.
Recent studies looking at various neuropsychological batteries have in general not shown any permanent brain damage from the long-term use of marijuana. According to Harrison Pope, a meta-analysis of such studies on long-term marijuana users found no brain damage in seven of the eight areas and only a small effect on the remaining area, learning.
Another meta-analysis of fifteen such studies of long-term users done by Igor Grant, who has been working in this area for a long time, concluded that marijuana did not cause brain damage. They did, however, find a “very small” impairment in learning and memory, but apparently did not feel it was significant enough to warrant being called brain damage.
A typical study, here, found deficits in current users, but none in former users, half of whom had been smoking 5 or more joints a week. The abstinence period was three months.
A study of male identical twins from the Vietnam Era, one of whom had formerly used marijuana and another of whom did not, found an absence of marked effects on cognition. A Canadian study found no permanent lowering of IQ scores in former heavy cannabis users.
Nevertheless, it also found that smoking more than 5 joints a week lowered one’s IQ by about 4.1 points. But smoking 3 joints a week or less raised IQ by 2-6 points, the same as abstaining from marijuana. The test measured IQ taken between ages 10-19, and the IQ rise was a consequence of going through adolescence.
Yet another study found that varying degrees of marijuana use from light to heavy did not accelerate cognitive decline in a group of 1200 persons of all ages. The only thing that did accelerate cognitive decline in this group was increasing age. Cognitive decline began as early as the 30′s and accelerated markedly in the 60′s.
Although permanent effects are dubious, there are residual effects for heavy users on memory, learning and recall. These effects are quite clear even 24 hours after use and in daily users, last, subtly, for up to a week after use and may be related more to a mild withdrawal syndrome, since they peak at about 3-5 days post-use. By one month post-use, though, these deficits are gone.
Therefore, chronic heavy users will always be somewhat impaired in these areas as long as they are using marijuana.
A 1995 study by Harrison Pope tested chronic heavy cannabis users against controls after one, seven and 28 days abstinence. Effects were seen at one and seven days, but after 28 days, there were virtually no differences between the groups.
This study shows that heavy long-term users will still be impaired even a week after their last joint. Yet it also shows that impairments are related more to recent use than to permanent damage.