Is tolerance unavoidable?
In general receptor-binding substances cause an "overflow" of your own already produced neurotransmitter. Your brain reacts by reducing the receptors. Also your own neurotransmitter production may be down-regulated by negative feedback. When you take away the "medication" afterwards, there are less receptors together with less neurotransmitter, causing withdrawal symptoms.

When you put in more substance in a control loop, there will always be some sort of "adaption" in order to reinstate a new balance (1). If you take in just enough drugs to balance the neurotransmitter-receptor system in a way, that the signal transduction in terms of nerve pulses remain the same, you have reached a tolerance stage, but not an effect stage. Still, medication is not "useless", since not all receptors will be dissipated. It is a matter of dose rate to increase the nerve pulses; once there is relatively more neurotransmitter (2) available than before medication, one should obtain a constant effect or an effectively altered control loop balance.

(1) https://books.google.at/books?id=z9TUch7m144C&amp;pg=PA34&amp;lpg=PA34&amp;dq=receptor+regulation+up+down+neurotransmitter+drugs&amp;source=bl&amp;ots=2-4GRChqBi&amp;sig=6e0YM-UrfgnMlw6e-rXR2Or7Qxc&amp;hl=de&amp;sa=X&amp;ved=0ahUKEwixr_D25ZDMAhWB7RQKHcF9DqYQ6AEIHzAA#v=onepage&amp;q=receptor%20regulation%20up%20down%20neurotransmitter%20drugs&amp;f=false

(2) http://www.merckmanuals.com/professional/clinical-pharmacology/pharmacodynamics/drug-receptor-interactions