Drug Dependence, Tolerance & Addiction

Introduction & terminology

• Dependence — an altered physiological state from repeated drug administration that requires continued drug presence to maintain equilibrium; cessation produces a characteristic withdrawal (abstinence) syndrome. Formerly "physical dependence"; reflects neuroadaptation. Typical of CNS depressants — opioids, alcohol, barbiturates, benzodiazepines.
• Addiction — a pattern of compulsive drug use in which procuring/using the drug takes precedence over all else and continues despite known harm. Formerly "psychological dependence." Dependence is not essential to addiction — amphetamines, cocaine, cannabis and LSD addict with little/no physical dependence.
• Substance use disorder (SUD) — the DSM-5 umbrella term that collapsed the old "abuse + dependence" split into one mild–moderate–severe continuum; diagnosis rests on a pathological behaviour pattern clustered into impaired control (incl. craving), social impairment, risky use and pharmacological criteria (tolerance, withdrawal).
• Reinforcement — a drug's capacity to produce effects that drive repeat use; the faster a drug acts, the more reinforcing it is — so IV/inhaled routes are most reinforcing. Opioids and cocaine are strong reinforcers; benzodiazepines weak.
• Drug abuse vs habituation — abuse = use deviating from approved medical/social norms (for regulators, any illicit-drug use); habituation = mild involvement with minimal withdrawal. KDT advises dropping "habituation" as a separate term — the difference from addiction is only quantitative.
• Tolerance, dependence and withdrawal are biological, not synonymous with addiction — a patient on correctly-dosed opioid or β-blocker shows tolerance/dependence/withdrawal without being addicted; prescribers must distinguish physiological dependence from SUD.