Are there undisputed benefits from prohibition regimes and/or undisputed harms from legalization/regulation regimes?

The question in the title of this post is a variation of the topic that consumed discussions today in my "Marijuana Law, Policy & Reform" seminar.  Though I am not sure I did a great job steering the discussion, I am sure that students had a number of interesting and thoughtful reactions to the first part of this question.  

One of my goals in these questions is to explore whether, if we take as an empirical given that prohibition regimes do tend to reduce use of a prohibited product, folks would accept or resist the contention that reduced use is an undisputed benefit of prohibition regimes.  (We have been considering the history of alcohol Prohibition in the US at the start of the course, and some research suggests that the enactment of the 18th Amendment and related laws drove down alcohol consumption to 30% of pre-Prohibition usage, although over the period of Prohibition alcohol use rose to reach about 60 to 70% of pre-Prohibition usage levels.)  Of course, to say reduced use of a product is an undisputed benefit indicates a belief that any and all use is an undisputed harm.   I suspect many folks would now resist the claim that any and all use of alcohol is an undisputed harm, and I explored with students whether they thought more of society was coming to resist the claim that any and all use of marijuana is an undisputed harm.

At the end of the class, we only briefly got to the question of what undisputed harms might be said to flow, at least in part, from legalization/regulation regimes for a drug like alcohol.  I started this part of the discussion with my concerns about significant harms that result from drunk driving, and other students raised issues related to crimes and physical violence and related to poor allocation of societal resources.  How much of these harms could and should be attributed to our current legalization/regulation regime for alcohol is, of course, a contestable question, but in this setting the issue seems to be not whether these matters count as harms, but rather whether a legalization/regulation regime increased or increases these harms.