Many people are concerned that if drugs were legalized, they'd be advertised. In America, this seems unlikely for two reasons: Congress would never permit such advertising, and any corporation that marketed psychoactive drugs commercially would be sued into bankruptcy.
Corporations meeting federal standards might be granted limited immunity from damage suits if they sold such drugs to the government for distribution though federally licensed outlets.
Yet other, and perhaps more potent, drugs would continue to be manufactured and sold on a black-market basis. Whether or not such a situation would reduce crime is not clear.