The effects of marijuana on the brain may be more complicated than experts previously thought, and may depend on factors related to the person using the drug, such as their genetics, two new studies suggest.


To assess whether smoking marijuana has any effect on brain volume.


After the examination of several hundred brains of siblings, and comparing those who smoked versus does who did not, marijuana users were found to have smaller amygdalas (area in the brain) than those who did not smoke. However, brain volumes did not change between twins or siblings, even if one had smoked marijuana and those who did not.


Authors concluded that there may be genetic factors that influence amygdala size – but there was no effect with relation to brain volume.