It is nothing like functional alcoholism. Marijuana does not cause someone to become overconfident and make stupid decisions and take risks, and it doesn't cause any significant change in reaction time when used regularly. But, of course, a functional alcoholic would probably drive drunk slightly more safely than someone who rarely drinks. But the direct CNS effects of alcohol make alcohol a pretty direct safety risk in ways that marijuana is not.

Also, do you support the .08 legal limit for alcohol? That's about two drinks for most people. If you do, do you think Marijuana should be any different? Why could I have two drinks, but not two joints?