Legalization means that the drug could then be sold in stores with increased distribution, lower cost and easier access. Legalization eliminates obstacles currently with it being illegal - fear of breaking the law, not easy to find for the average person, social stigma, etc. With those and similar obstacles removed more people will use it. In addition, over time it would become more socially acceptable.
So there might not be a black Friday rush on it but usage would increase over time. Also, it isn't just the use of the drug alone but when used with others. That is why caffeinated alcohol drinks were outlawed.
All legalization does is increase the problem and move it around from interdiction to cleaning up the dead, injured and impaired after the harm is done.