Educate people on the effects of all substances, including tobacco and alcohol and the results of using (including that cocaine usually rots the nose requiring plastic surgery) and then legalise the lot. Anyone with an ounce of sense would not use drugs if they knew what they do.
Result: Saves police, court etc. time, thus public money and just about removes all the criminal problems associated with them when they are illegal..
The illegality of prostitution should also be removed for the same reasons.