Andrea Barthwell quotes:

By characterizing the use of illegal drugs as quasi-legal, state-sanctioned, Saturday afternoon fun, legalizers destabilize the societal norm that drug use is dangerous. They undercut the goals of stopping the initiation of drug use to prevent addiction.... Children entering drug abuse treatment routinely report that they heard that 'pot is medicine' and, therefore, believed it to be good for them.

It is not a medicine. You don't know what's in it. If there were compelling scientific and medical data supporting marijuana's medical benefits that would be one thing. But the data is not there.