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"Persistent users" of marijuana - those that consistently use four or more times per week - show a fairly significant cognitive deficit later in life. The apparent damage is irreversible if heavy use begins in adolesence. Those who began heavy use in college and did not use for at least a year prior to testing showed no deficit.
This is yet another study challenging the presumption that cannibis is a relatively innocuous drug. This mistaken belief is actually becoming more prevalent among people in the US - especially the young. The article notes that this is the first year that US high school students are more likely to use marijuana than tobacco.
The study suffers from many of the typical limitations associated with this sort of research. Although they found evidence of a relationship between early cannibis use and IQ, we don't know what the actual cause of the decline is. Some may ask "does cannibis use lead to lower IQ or does lower IQ lead to cannibis use?" - but given the pattern of results, it seems more likely that the cause is cannibis use or another factor associated with cannibis use (especially given that they looked at just one cohort).
Regardless, it's likely that the study will be welcomed by those who question the use of cannibis -- and discounted by those who defend it (especially among those that used heavily in adolescence
).
