As long as millions of people desire drugs, they will be supplied. When police are most effective, they bust many dealers and confiscate pounds or even tons of drugs. Then the price on the street goes up. Laws of supply and demand deem purchases will decline when price increases, even for addictive substances. But as price increases so does property-related crime.
Drugs cost more so holdups and burglaries increase.
Detroit once had a very unofficial experiment of not busting heroin dealers for a period of time. Also violent crime increases when drug possession is criminalized. Milton Friedman, a conservative, Nobel Prize-recipient economist, has demonstrated that during Prohibition homicides rose dramatically due to fighting among gangs over areas for exclusive distribution of alcohol. This is how organized crime spread from Sicilian neighborhoods to around the nation. After alcohol's relegalization during the New Deal, homicides dropped dramatically.