Most substances affect the brain’s “reward circuit” with the chemical messenger dopamine.
Dopamine is a type of neurotransmitter. Your body makes it, and your nervous system uses it to send messages between nerve cells. That is why it is sometimes called a chemical messenger.
This reward system controls the body’s ability to feel pleasure and motivates a person to repeat behaviors needed to thrive, such as eating and spending time with loved ones. This over-stimulation of the reward circuit causes the intensely pleasurable “high” that can lead people to take a drug repeatedly.
As a person continues to use substances, the brain adjusts to the excess dopamine by making less of it or reducing the ability of cells in the brain’s reward circuit to respond to it.
This reduces the “high” that a person feels compared to the “high” they felt when first taking the drug.
This effect, known as tolerance, means the person might need to take more of the drug to achieve the same dopamine high.
As individuals continue to misuse substances, progressive changes, called neuroadaptations, occur in the structure and function of the brain.
These neuroadaptations compromise brain function and drive the transition from controlled, occasional substance use to chronic misuse, which can be difficult to control.
More than 60 percent of people treated for a substance use disorder experience relapse within the first year after they are discharged from treatment.