According to some studies, long-term cannabis abuse may cause abnormal brain structure and poor memory, especially in people who have or are at risk of developing schizophrenia. Cannabis, particularly THC (the main psychoactive component of cannabis), may also reduce REM sleep, which is the stage of sleep when we do our most active dreaming, processing emotions, and cementing new memories. Decreasing REM sleep may have some benefits for people with PTSD, since nightmares are a common and disturbing symptom. However, for most people, poor sleep may impair cognitive performance and focus, and increase the risk of cognitive decline and dementia in the long term. Therefore, it is possible that years of poor sleep due to cannabis abuse would contribute to some brain damage.

However, these effects may depend on several factors, such as the dose, frequency, duration, age of onset, and individual susceptibility of cannabis use. Some studies have found no significant effects of cannabis on brain structure or cognition after controlling for confounding variables. More research is needed to establish a causal link between cannabis abuse and brain damage.