With origins in bigotry and racism, and sustained by the use of force, drug prohibition is a violation of the most basic human rights.
Prohibition leads to more dangerous drugs, and has a "forbidden fruit" effect, but whether lax or tough has little effect on patterns of drug usage.
Although intended to protect people from drugs, prohibition worsens the situation of drug users, increasing criminality and health problems.
Prohibition enriches the worst criminals and leads to more violence.
Due to prohibition's nature, drug enforcement inevitably leads to overburdened courts and jails, fewer resources devoted against violent crime, and corruption at all levels.
Faced with failure, prohibitionists seek more and more infringements of individual liberties, from restricting speech to banning guns, from more intrusive searches to higher taxes.
Prohibition discourages promising drug research, denies medically proven uses of recreational drugs, and eliminates beneficial industrial uses of drug sources. Relying on an image of evil drugs, prohibition propaganda spreads ignorance of the true effects of drugs.
As prohibition's failure becomes evident, voices from all points on the political spectrum speak out against it, from Nobel Prize-winning economists to conservative columnists and former police chiefs.
For more information:
Basic
Facts about the Drug War: excellent question-and-answer
introduction to prohibition
Erowid: one of the best
collections of information about psychoactive substances
The Drug Library:
DRCNet's "largest online library of drug policy"