Drugs that work via the dopaminergic system (often referred to as the reward system in the brain) to increase levels of the neurotransmitter dopamine in an area of the brain called the nucleus accumbens seem to have an addictive potential.

The speed at which dopamine accumulates and the level it accumulates to appear to determine level of addiction risk associated with individual drugs. Route of administration also plays a part.

Drugs that are injected or inhaled enter the brain more quickly than those taken orally and therefore tend to be associated with a greater risk of addiction.⁴

With continued use of a substance the reward sensation changes to an expected reward. At this point dopamine neurons fire in response to stimuli that are associated with the reward stimulus that comes from taking the substance.

The phenomenon is known as conditioning: stimuli such as objects, environments, routines or emotions that predict the reward are referred to as drug cues. Further substance use strengthens the conditioning, which in turn promotes further drug taking.

That may be the reason that people with substance use disorder undertake risky, illegal or unhealthy behaviours just to obtain their next drug reward.

Other neurotransmitters in the brain such as the GABAergic, opioid, endocannabinoid, cholinergic, serotonergic and noradrenergic systems are also affected and lead to changes in mood, cognition, sleep, and stress reactivity.⁴