Because of the direct effect of illicit substances on neurotransmitter action, there is often a dramatic alteration of neurotransmitter receptor activity. For example, in response to foreign chemicals that mimic the activity of neurotransmitters, the neurons may decrease their own neurotransmitter production or activity. Receptor sites may die off. This change of the actual structure of the neurons contributes to the addictive process. When the brain makes less neurotransmitter or is less able to \2 process it, craving sets in. Drug tolerance, the need for more and more of the same drug to achieve the same psychological effect, is likewise related to the changes in the structure of the neuron.

The list below shows which drugs affect which brain chemical systems:

• Marijuana: Cannabinoid receptors
• Cocaine: Dopamine
• Heroine: Opioid receptors
• Ecstasy: NMDA Glutamate receptors
• Alcohol: GABA