WHY WOULD THERE BE MORE CRIME IF THE SUPPLY OF DRUGS FELL?
What the above answer says is about half correct. Here's more or less the same answer in economic terms and then the other half of the story.
If the supply of drugs fell, then as you know the equilibrium price of drugs would go up.
This would likely lead to drug dealers being able to get more profit for their drugs. It would also lead to greater competition for the supply of drugs that still remained.
In both cases, drug dealers would be more motivated to get drugs and would probably commit crimes fighting over the drugs.
The other half of things is that users would need to get more money to pay for their drugs. Once you're addicted, the price elasticity of demand for drugs is very low -- you buy no matter the price.
So drug addicts would have to steal more things to pay for their more expensive drugs.
This questions assumes that there will be more crime. I am not certain that this would happen. If drug supplies went way down, then, of course, people would pay more and may even do crazy things to get the money necessary to buy.
However, there are other possibilities. If drugs became extremely scare, perhaps the average drug user would no longer take drugs at all, because they cannot get their hands on it. Drugs might be a practice only for the elite and extremely wealthy. In this sense, crimes may actually go down. In short, I do not think we can assume that crimes would increase with the smaller supply of drugs.