A large longitudinal study in New Zealand found that persistent marijuana use disorder with frequent use starting in adolescence was associated with a loss of an average of 6 or up to 8 IQ points measured in mid-adulthood.42 Significantly, in that study, those who used marijuana heavily as teenagers and quit using as adults did not recover the lost IQ points. Users who only began using marijuana heavily in adulthood did not lose IQ points. So if you want to buy weed online in canada for example, then as an adult it should not affect your IQ. These results suggest that marijuana has its strongest long-term impact on young users whose brains are still busy building new connections and maturing in other ways. This is probably why there are some concerns even when marijuana is used for medical purposes. There are plenty of places to get medical marijuana from, for example you could go to a Michigan dispensary. Despite this, there are still some concerns as the endocannabinoid system is known to play an important role in the proper formation of synapses (the connections between neurons) during early brain development, and a similar role has been proposed for the refinement of neural connections during adolescence. If long-term effects of marijuana use on cognitive functioning or IQ are upheld by future research, this may be one avenue by which marijuana use during adolescence produces its long-term effects.43
However, recent results from two prospective longitudinal twin studies did not support a causal relationship between marijuana use and IQ loss. Marijuana users did show a significant decline in verbal ability (equivalent to 4 IQ points) and in general knowledge between the preteen years (ages 9 to 12, before use) and late adolescence/early adulthood (ages 17 to 20). However, at the start of the study, future marijuana users already had lower scores on these measures than future non-users, and no predictable difference was found between twins when one used marijuana and one did not. This suggests that observed IQ declines, at least across adolescence, may be caused by shared familial factors (e.g., genetics, family environment), not by marijuana use itself.44 It should be noted, though, that these studies were shorter in duration than the New Zealand study and did not explore the impact of the dose of marijuana (i.e., heavy users) or the development of a cannabis use disorder; this may have masked a dose- or diagnosis-dependent effect.