As tolerance for the drug increases, larger doses are needed to produce the same effect on consciousness. Therefore, the addicted person's body chemistry becomes progressively more tied to the drug. (Lahey, 1997 p. 180).
The area that that is directly affected during physiological addiction can be found in neurotransmitters.
Understanding how neurotransmitters fit receptors can explain the actions of certain drugs. When a drug alters the activity of a neurotransmitter on a postsynaptic neuron, it either halts or enhances synaptic transmission. A drug that binds to a receptor, blocking a neurotransmitter from binding there, is called an antagonist. A drug that activates the receptor, triggering an action potential, or that helps a neurotransmitter to bind, is called an agonist. The effect of a drug depends upon whether it is an antagonist or an agonist; on the particular behaviors the affected neurotransmitter normally regulates; and in which parts of the brain drugs affect neurotransmitters and their binding to receptors. (Sheir, Butler, Lewis, 2002).
The primary areas of abuse for addicts are called psychotropic drugs. Psychotropic drugs are defined as a class of substances that alter conscious experience. The major types are: Depressants, Stimulants, Hallucinogens, Inhalants and Marijuana. .
The categories of depressants and stimulants frequently show the majority of drug addiction. While all categories lead to both physiological and psychological dependence, depressants and stimulants show the highest percentage of abuse. Both categories are subdivided into a multitude of different types.
Depressants include tranquilizers, narcotics, and sedatives. Examples of tranquilizers are Valium and Xanax. Examples of narcotics are opiates (Codeine, Heroin and Morphine) as well as newer synthetics such as Demerol and Percodan. Examples of sedatives include Barbituates and the more socially acceptable Alcohol.