as long states can make their own laws and not bound to ask the federal government authorization first, they sure can. it is only a question if the fed gov will obey the state laws as federal law differs and this part is highly unlikely as they don't do so in other states with MM laws.
besides, do they want to introduce stiffer penalties for prescription drug abuse too - if not they sanction one drug unconstitutional compared to others, not to mention that prescription drugs do harm and are often very addictive which marijuana isn't as science has proved many times conclusively.

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