The harmful effects of prohibition exist irrespective of the drug. History has shown that they are largely of the same character (but perhaps different in scale) for cocaine, heroin, alcohol, etc.

You take a substance out of the hands of professionals and place it into the hands of criminals. The quality of the substance goes down. The number of dead bodies goes up. The truth about the drugs themselves gets completely distorted with lies and propaganda instead of becoming common knowledge.

Legalization should go hand-in-hand with education and healthcare reform. If you get those elements right, you'll have a much more functioning society. Fewer drunk drivers, fewer overdoses, and no drug cartels to speak of.