FOLLOW THE MONEY
Many of the supporters of the drug war are special interests whose profits, power, prestige or employment depend on the continuation of the drug war.
Back in 1972, The National Commission on Marihuana and Drug Abuse [aka The Shafer Commission], appointed by President Nixon, was alarmed by the influence of money on policy saying that the result of our drug policy:
"...has been the creation of ever-larger bureaucracies, ever-increasing expenditure of monies and an outpouring of publicity so that the public will know that 'something' is being done. Perhaps the major consequence of this ... has been the creation of a vested interest in the perpetuation of the problem among those dispensing and receiving funds ... In the course of well-meaning efforts to do something about drug use, this society may have inadvertently institutionalized it as a never-ending project."
The amount of money spent in 1972 that so disturbed the Commission was a small fraction of the amount spent today.
When the New York City Bar Association finally issued its report on drug policy in 1994, after 8 years of intermittent study, they were similarly concerned:
"Large sums of tax dollars create enormous self - perpetuating bureaucratic agencies. They have ample motivation to exaggerate or distort the extent and danger of 'drug abuse' so as to justify their existence. Inherently biased, they have great potential to ignore the public's true welfare."
As usual, Milton Friedman  covered the problem succinctly :
"I find it almost incredible how people can support the present system of drug prohibition. It does so much more harm than good."
[Interviewer: Why do they ?]
"Very good question. And the answer is because there are so many vested interests that have been built up behind the present drug war."
In 1997, Federal Judge John Kane emphasized the link between special interests and the misinformation that has promoted mythology:
"Flawed studies and statistics are used to promote whatever policy is in vogue.
"The result is waste and nonsense that in any other human endeavor would be intolerable.
"In sum, truth takes a holiday, and special interests burrow into the sources of wealth and influence.
SPECIAL INTERESTS AT WORK
" The problem to be confronted is that too many U.S. elected officials and employees of public agencies have incentives to conduct a war on drugs, to ignore or resist policy alternatives, and most assuredly not to explore alternatives that might more effectively address the problems generated by substance abuse.
"Treatment providers also have an incentive to advocate harsh penalties for drug use, because they generate demand for their services through court ordered treatment and private spending for treatment driven by the fear of punishment.
"Most influential players in the formulation of drug policy, and all the dominant ones such as legislators, police agencies, and prosecutors, have incentives to conduct a war on drugs even though more effective policies are readily available.
"Enforcement dominates drug policy not because it is the most effective policy, but because it best serves the interests of the people who work in the institutions that dominate the drug policy decision-making process.
"Drug war, the excessive application of enforcement that aggravates rather than mitigates the social consequences of drug use, is waged because it is in the interests of particular politically influential groups, including law enforcement bureaucracies and public officials.
"A policy can fail completely, while at the same time entrepreneurial bureaucrats expand their reputations and end up being substantially better off. As Breton amd Wintrobe suggest, however, 'One need not assume Machiavellian behavior, deceit, or dishonesty on the part of bureaucrats, because in all likelihood the pursuit of their own interest will be, as it is for everyone else, veiled in a self-perception of dedication and altruism.'"
- from RATIONALIZING DRUG POLICY UNDER FEDERALISM by David W. Rasmussen (Professor of Economics and Director of the DeVoe L. Moore Center at Florida State University and Bruce L. Benson: Distinguished Research Professor of Economics at Florida State University. (Ph.D., Texas A&M University, 1978)
For a small part of an excellent article on the growth of "The Prison-Industrial Complex, " see 
For an example from the advertising industry (after government reports show "... youths who had seen or heard drug/alcohol prevention messages outside of school in the past year were somewhat more likely to report past year marijuana use than those who had not been exposed." ) see 
For media outlets, see 
For pharmaceutical companies, see 
For California, see