The most harmful drug is belief.
Belief, if qualitatively described as an electro-chemical reaction in the mind that influences behavior, is a very powerful drug.
Belief has caused entire nations to kill other nations.
Belief is what causes people to turn to terrorism, because they believe they are doing the 'right' thing.
Belief caused Hitler to rise to power, his people believed in him.
Belief gives racists their power, they believe they are better because of their skin.
Belief even causes drug addiction, the belief in the pleasure being worth the consequences, or that there are no consequences.
So what is the most dangerous drug of all? Belief. Believing. Faith.
The original drug that remains in us all, our entire existence.