To make sale, possession, and consumption of drug X illegal. Of course, people who want drug X will continue to purchase it on the black market. When the State sees the consumption of X increase, it increases penalties for those violating the prohibitions. The price of X increases and with it the potential profits possible for suppliers. Competitors for customers cannot legitimately offer cheaper prices or better service to increase market share.
Disputes occur as they struggle for domination.
With no courts to resolve their differences, the suppliers ultimately must resort to physical violence to win. Innocent people are often caught in these fights. They also see their neighborhoods deteriorate as drug users and dealers move in and decent people who can afford to do so move out. But, of course, criminals will not obey gun laws if they are unafraid to commit murder. Calls for gun prohibition, registration, or outright confiscation mount.