All drugs, whether from a natural or laboratory source, are chemicals, and as such they network into the central nervous system (connected to the brain by the spinal cord) and change the way nerve cells process information.
Some drugs, for example marijuana or heroin, can activate nerve cells by mimicking the chemical structure of a natural neurotransmitter. The similarity deceives the nerve receptors, permitting the drugs to attach to and activate the neurons. Since these drugs have a similar structure, but a dissimilar process, they activate neurons differently from the manner of a natural neurotransmitter, use of these drugs appears to result in abnormal messages being sent via the brain and the nervous system.
Other drugs, for example amphetamine or cocaine, can cause neurons to release unprecedented amounts of natural neurotransmitters or to inhibit their normal life cycle. This artificially-induced release of bio-chemicals results in a message greatly amplified or distorted, which ends up overwhelming the nervous communication channels, producing the well-known associated physical and psychological reactions.
All drugs of abuse directly or indirectly target the brain and central nervous system by releasing from twice to ten times the usual amount of dopamine that natural stimuli would generate, and the effects of such a prodigious emission can last much longer than those produced naturally.
Dopamine is a neurotransmitter found in those parts of the brain that regulate movement, emotion, cognition, motivation, and pleasurable sensations. The over-stimulation of the brain in this manner is what creates the euphoric effects sought-after by people who use drugs for recreational purposes, and can teach them to repeat the behavior again and again.
The impact of the release of normal levels of dopamine on the reward circuit of a drug user’s brain, due to the manipulation of its level so many times, can become abnormally low, and thus the ability to experience any natural or ordinary pleasures is reduced. This is why addicts begin to feel dull and lifeless, before entering a kind of numbness, and a state in which only the drug of choice brings any degree of pleasure. Beyond this stage, the addict requires increasingly greater amounts of the drug just to bring their dopamine function up to normal. This is referred to as drug tolerance.
Chronic exposure to drugs or alcohol negatively impacts the way the brain interacts with control behavior, which is behavior specifically related to drug abuse and addiction. In the same way that continued abuse leads to tolerance, it may also lead to addiction and dependence.