Once again, the "War on Drugs" must be judged by its results rather than its intent. The intent might be to keep people from harming themselves by the use of drugs. While well-meaning, the result has been to waste an enormous amount of taxpayer dollars, needlessly imprison a lot of people for possession, and creating a criminal culture around the distribution of drugs.
Forget the taxing. Rather than adding one more form of government thievery, why not eliminate ten others. Legalize drugs for the simple reason that criminalizing them has been a disaster.
Here's a discussion of U.S. drug policy where Pete Wilson gets his ass handed to him by Milton Friedman.
http://video.google.com/videoplay?docid=8056316969143175615&q=Milton+Friedman+Pete+Wilson&hl=en