The act of prohibiting the use of drugs is agains't ethics by itself. One can do whatever he wants with his own property and individuality UNLESS he manages to cause damage on other people. If this is the case, then the prohibition isn't right since the worng act was not a direct consequece of drug
All human actions are controlled by our sinapses and hormone intervention. Both of them are obligatory a consequence of their ambient and birth properties(ex: genome), thus the cause of anything would be dragged to infinity until the beggining of our universe, that determined all.