Until I found concrete confirmation of this theory.

Good thinking. My opinion is that drugs are illegal because they completely destroy a person. Morally and physically. It is not beneficial for the state to have useless slaves.

Another thing is alcohol. Alcohol is a legal drug with the consequence that we can be easily acquainted. People who constantly drink alcohol become weak-willed. But he will help to relax. Alcoholism and drug addiction are almost impossible to win.
The only person in history who completely conquered drug addiction in his country was Adolf Hitler.