Several studies, including two large longitudinal studies, suggest that marijuana use can cause functional impairment in cognitive abilities but that the degree and/or duration of the impairment depends on the age when a person began using and how much and how long he or she used. 38,39 Other studies have not found significant structural differences between the brains of people who do and do not use the drug.

Some studies suggest regular marijuana use in adolescence is associated with altered connectivity and reduced volume of specific brain regions involved in a broad range of executive functions such as memory, learning, and impulse control compared to people who do not use. Imaging studies of marijuanaвЂ™s impact on brain structure in humans have shown conflicting results. 35вЂ“37 Studies in rats also show that adolescent exposure to THC is associated with an altered reward system, increasing the likelihood that an animal will self-administer other drugs (e.g., heroin) when given an opportunity (see ' Is marijuana a gateway drug?').

32вЂ“34 Cognitive impairments in adult rats exposed to THC during adolescence are associated with structural and functional changes in the hippocampus. Rats exposed to THC before birth, soon after birth, or during adolescence show notable problems with specific learning and memory tasks later in life. Substantial evidence from animal research and a growing number of studies in humans indicate that marijuana exposure during development can cause long-term or possibly permanent adverse changes in the brain.

The brain