As drugs are illegal, they are being marketed by outlaws, who charge an inflated price for them. Much more inflated that the amount it requires to grow, manufacture and distribute them. As a result, there is a growing black market for the distribution of drugs and drug users eventually need to commit crimes like theft and robbery to support their habits.
The average crime rate in the U.S increased twice fold due to the prohibition on Alcohol. Once the prohibition was lifted, it decreased back to its original state. It is expected that drug-induced crimes cause a similar increase, due to the similar nature of this prohibition.
In this analysis one should also include the very “crimes” of drug use, trafficking, and sale, as well as crimes caused due to the criminal nature of some drug dealers. Drug trafficking is also a huge source of income for Organised crime.
All of this crime can be eliminated in a swift blow by legalising drugs. When drugs will become legal, they will be produced, distributed and sold by local legitimate entities, who will be subject to Capitalistic market constraints. Their prices will drop to a minimum, allowing everybody to buy them at the needed quantities. Criminals will no longer be able to make a decent living from selling them in the black market (as there will be none.).