Yeah, access to it by problem drinkers. I think you ought to have to have a license (a checkbox on your drivers license will do) that allows you to buy it.
You get caught abusing it, and they revoke your ability to legally purchase it. The same system would work for Marijuana.
There are enough sensible drinkers to keep the market focused on them rather than chase after the smaller number of problem drinkers.
If the black market isn't worth the trouble, people won't bother feeding it.