"Heavy marijuana use" suggests Joint-$moking (and the article offers no details about how the cannabis for this study was ingested).   A Joint is a drug cocktail of heat shock, carbon monoxide (CO) and 4221 Combustion toxins known to cause pathologies conveniently attributed to the cannabis.  

Recent studies have in fact suggested that "heavy" tobacco $igarette $moking produces cognitive deficits such as those here linked to cannabis.  The key fact is that today VAPORISERS and flexdrawtube one-hitters (25 mg per serving instead of 500) exist which eliminate the monoxidal "heavy use" $moking hazards-- but PROHIBITION creates conditions under which the Joint remains popular because it is easier to HIDE from your Mom or the cop, or quickly used up thus disposed of at later health cost.  And municipalities have had "Paraphernalia Laws" which make it hazardous to possess or be caught with safer alternatives to $igarette paper.

The greatest "danger" with cannabis legalization is that it will bring with it legalization of vapes, which may spread over into the tobacco-using population, destroy the $igarette "industry" worldwide and save 6,000,000 lives a year.