Functional tolerance refers to changes in the post-synaptic synapses of the central nervous system (CNS) that are caused by abnormal exposure to endogenous (internal) and exogenous (external) chemicals, particularly hormones and illegal drugs. In simpler terms, this refers to the way the body can build a tolerance or resistance to foreign substances.
This is why, in the case of drugs and/or alcohol, a person constantly needs more to achieve the same effect that they felt the first time they used it. In drug parlance this is called ʺchasing the dragonʺ because although you try to achieve the same ʺhighʺ or ʺbuzzʺ you never quite get there and keep increasing dosages.
Functional tolerance can cause medically dangerous situations as individuals sometimes keep on increasing dosages until they reach a point where it is lethal and they overdose, which can cause death.