THE NEUROSCIENCE OF ILLICIT DRUGS: A REVIEW ON THE NEUROBIOLOGICAL MECHANISMS AND BRAIN EFFECTS 

Abstract

Background: Illicit drug use is an increasing global public health problem that causes significant neurological harm, psychiatric impairment, and decreased lifespan. Stimulants (e.g., cocaine, methamphetamine), psychoactive drugs (e.g., cannabis), and opioids (e.g., heroin) cause neurological harm at the level of brain structure, neurotransmitter availability, and neural processes that cause persistent changes in cognition and behavior. Method: This PRISMA-ScR-guided review synthesizes studies (2015-2025) related to brain change associated with major illicit drugs, using multiple databases. Only studies with direct human or animal evidence of neurobiological alterations were included. Outcomes were categorized by drug category and regions of the brain affected, as well as impacted neurotransmitters or mechanisms. Result: Methamphetamine, cannabis, cocaine and heroin, destroys different regions and systems like; dopaminergic and serotonergic systems by oxidative stress, and also disrupts the blood-brain barrier. Across and within drugs, both reversible and irreversible drug-related changes appear in the brains of drug users. Conclusion: Illicit drugs have complex and geographically determined => neurotoxic effects on neurotransmission, brain networks, and cognitive control. These mechanisms reposition addiction as a neurobiological disorder (not a moral failing) and make clear the need for regionally determined preventative, therapeutic, and policy responses to limit long-term neuroharm.