The criminals in prison for drug related offenses are there for dealing drugs, not using them. Don't get too worked up for these scumbags who would sell any drug to any person of any age just to make a buck.
If you're in favor of legalizing all drugs (including prescription drugs) in order to take away the profit from these gangs, that's your prerogative. I don't agree.
I do not favor the use of such hideous chemicals but no one has been legitimately appointed the guardian of adults who choose to poison themselves. If you believe that you are not responsible enough to make such choices for yourself that's your analysis. I am worthy of freedom and object to others deciding for me and making me pay for the enforcement.
The demand side of the market is essentially fixed in size, so for a brief period there would be an increase in drug-turf violence while those gangs who have other 'wares' to peddle attempt to expand their zone to a size sufficient for financial sustinance.
Even illegal trade is subject to market principles.
Prior to about 1911, the US did not have any drug laws. You could buy opium, cocaine, or whatever. The concept of things requiring a doctor's prescription is also a 20th Century invention. Yet we did not have a massive drug problem in the pre-20th-century. Why is that?
My speculation is that the incentive structure was different. If you took drugs or alcohol to the point to where you were unable to work, you went hungry. Eventually you might starve. Sooner than that, your family would leave you. This was an incentive to not go that far.