All drugs will eventually cause alterations in the brain, as the brain is a 'plastic' organ.
I read a book once that included a chapter on the very subject the OP is inquiring about.
The study was looking at tests of cognitive function, an indirect approach of analyzing brain pathways.
I remember these specifics: two groups were tested, one had been smoking regularly for 10 years and the other for 30 years. Upon discontinuation of cannabis, the first group eventually returned to 'control levels' for short-term memory, among other cognitive functions....but it took up to a year.
In the second group, the alterations seemed to be permanent. Abstinence from THC didn't seem to reverse the changes. Of course, you must consider the fact that anyone who smokes for 30 years, is likely to be 40-50 years old during the study. Age itself is obviously a factor. Combine age with THC and you should expect some permanent changes. Perhaps regular use of THC should be limited to your younger years only...
More significant than this - cannabis is associated with a BIG increase in the onset of psychiatric disorders. A very large scale military study shows a 500% increase in the occurrence of schizophrenia among cannabis users.
This does not mean weed causes it, but it DOES set it off in susceptible individuals.
If you are genetically or environmentally susceptible, smoking marijuana greatly increases your odds of developing the disorder.
Since recovering from MDMA use, I have found weed causes sudden and drastic increases in anxiety - occasionally to the point of feeling like i'm going to have a stroke! This is pretty rare, but even on the nights when it still gets me 'stoned', it is WAY different from what it used to be.
I scoff at the idea that weed is a 'gateway' drug, but after a decade of regular use, I strongly encourage younger users to moderate their intake. No drug should be used continuously for years, regardless of its supposed 'safety'.
Besides, taking breaks will lower your tolerance and clear your mind.
