Even occasional recreational use of marijuana can have a permanent impact on the brain...which researchers at Northwestern University say they can see.
Researchers there say they studied the brains of 40 young adults, half of whom smoked marijuana casually. They found the marijuana users had major differences in their brains, when compared to those who had never used the drug. The regions related to emotion, and motivation, were altered in marijuana users. And the more marijuana someone had used, the more pronounced the effect on the brain.
The study only looked for noticeable changes in the brain, not how these changes affected brain function.