Here is a fact that surprises most people: the majority of people who use cocaine, heroin, or alcohol never develop an addiction. Landmark epidemiological work in the 1990s estimated that roughly 15–20% of people who use these substances go on to develop the compulsive, out-of-control pattern of use that defines a substance use disorder (Anthony et al., 1994). The rest use, sometimes for years, without losing control.
That single observation reframes the entire question of addiction. The right question is not “why are drugs addictive?” but “why does the same drug capture one person and not another?”
Addiction is a property of the individual, not just the drug
Drugs of abuse all act on the brain’s reward circuitry, but exposure alone does not produce addiction. Two people can take the same substance, at the same dose, for the same length of time, and one will walk away while the other spirals into compulsive use. The difference lies in the biology of the individual: their genetics, their brain circuitry, their stress systems, their environment, and interactions among all of these.
This is why modern addiction science treats vulnerability and resilience as measurable biological traits. If we can identify what pushes an individual toward one outcome or the other, we can predict risk, intervene earlier, and design treatments matched to the person rather than the substance.
How do you study individual differences scientifically?
You cannot randomly assign people to become addicted. What you can do is study populations, human or animal, in which every individual is exposed under identical conditions, and then ask why outcomes diverge.
This is the strategy at the heart of our center’s research. We study large populations of genetically diverse laboratory rats (heterogeneous stock rats) given identical access to alcohol, opioids, or cocaine. Just as in humans, only a minority of these animals develop compulsive, addiction-like behavior, escalating intake, working harder and harder for the drug, and continuing use despite negative consequences (Deroche-Gamonet et al., 2004; Ahmed & Koob, 1998). The 15–20% figure seen in human epidemiology appears again, in animals that share our genes’ diversity but none of our social confounds.
Because every animal is phenotyped the same way and whole-genome sequenced, we can trace vulnerability to its biological roots: gene variants, brain-network signatures, gene-expression programs, and even signals from the gut.
What makes someone vulnerable?
No single factor determines who develops addiction, but research consistently points to several contributors:
- Genetics. Twin and family studies estimate that roughly half of addiction risk is heritable. Genome-wide studies in both humans and genetically diverse rats are now identifying the specific variants involved (more on the genetics of addiction).
- Brain circuitry. Vulnerable individuals show distinctive patterns of connectivity in circuits linking the prefrontal cortex, striatum, and amygdala, differences that whole-brain imaging can now measure directly (Kimbrough et al., 2020).
- The shift to “using to feel normal.” Addiction involves a transition from taking a drug because it feels good to taking it to escape feeling bad, a shift from positive to negative reinforcement driven by brain stress systems (Koob & Le Moal, 1997; Koob & Volkow, 2010).
- Body-to-brain signals. Emerging evidence implicates the gut microbiome and the vagus nerve in shaping mood, stress, and drug-related behavior (more on the gut–brain axis).
Why this matters
Treating addiction as a moral failing ignores the biology: some brains are measurably more at risk than others, in ways their owners never chose (Volkow et al., 2016). Pinpointing the mechanisms of vulnerability, and of resilience, is the path to precision medicine for substance use disorders: predicting who is at risk, and matching each patient to the treatment most likely to work for them.
References
- Anthony JC, Warner LA, Kessler RC (1994). Comparative epidemiology of dependence on tobacco, alcohol, controlled substances, and inhalants: basic findings from the National Comorbidity Survey. Experimental and Clinical Psychopharmacology 2(3):244–268.
- Deroche-Gamonet V, Belin D, Piazza PV (2004). Evidence for addiction-like behavior in the rat. Science 305:1014–1017. PMID 15310906
- Ahmed SH, Koob GF (1998). Transition from moderate to excessive drug intake: change in hedonic set point. Science 282:298–300. PMID 9765157
- Koob GF, Le Moal M (1997). Drug abuse: hedonic homeostatic dysregulation. Science 278:52–58. PMID 9311926
- Koob GF, Volkow ND (2010). Neurocircuitry of addiction. Neuropsychopharmacology 35:217–238. PMID 19710631
- Kimbrough A et al. (2020). Brain-wide functional architecture remodeling by alcohol dependence and abstinence. PNAS 117:2149–2159. PMID 31937658
- Volkow ND, Koob GF, McLellan AT (2016). Neurobiologic advances from the brain disease model of addiction. New England Journal of Medicine 374:363–371. PMID 26816013