It's been a bit of an open question whether legalised marijuana would lead to more or less alcohol use. If the two goods are complements, say if people liked drinking while consuming cannabis, then any increase in cannabis use could yield greater alcohol use. If they were substitutes and people smoked instead of drinking, alcohol use could drop.
So, legalising cannabis (which would lower the market price of cannabis) would have the effect of increasing cannabis use, and decreasing alcohol use. This is an important result (if backed up in other studies), given the increasing calls for legalisation in New Zealand and elsewhere.
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