If punishing buyers is more effective than punishing sellers, why don’t we place more emphasize on punishing buyers??

The Columbian Cartel makes it very clear, the problem the U.S. has to deal with is the demand for cocaine -- not the supply. If there is little or no demand for drugs -- there will be little or no supply. Demand creates its own supply . Would you agree ?

How would you test the hypothesis that being tough on users will cut drug use more than being tough on sellers?