Addiction Medicine

Addiction is a brain and mental disorder characterized by compulsive engagement in rewarding stimuli despite adverse consequences. However, in many ways, the nature and etiology of addiction remains a controversial topic;[1] addictions can also be viewed through a social and political lens as well.[2]

A substance use disorder is diagnosed when there is a problematic pattern of substance use leading to clinically significant impairment or distress, as manifested by at least 2 of the following, occurring within a 12-month period:

  1. The substance is often taken in larger amounts or over a longer period than was intended.
  2. There is a persistent desire or unsuccessful efforts to cut down or control the substance use.
  3. A great deal of time is spent in activities necessary to obtain the substance, use the substance, or recover from its effects.
  4. Craving, or a strong desire or urge to use the substance.
  5. Recurrent substance use resulting in a failure to fulfill major role obligations at work, school, or home.
  6. Continued substance use despite having persistent or recurrent social or interpersonal problems caused or exacerbated by the effects of the substance.
  7. Important social, occupational, or recreational activities are given up or reduced because of the substance use.
  8. Recurrent substance use in situations in which it is physically hazardous.
  9. Substance use is continued despite knowledge of having a persistent or recurrent physical or psychological problem that is likely to have been caused or exacerbated by the substance.
  10. Tolerance, as defined by either of the following:
    • (A) a need for markedly increased amounts of the substance to achieve intoxication or desired effect, or
    • (B) a markedly diminished effect with continued use of the same amount of the substance.
  11. Withdrawal, as manifested by either of the following:
    • (A) the characteristic withdrawal syndrome for the substance, or
    • (B) the substance is taken to relieve or avoid withdrawal symptoms.


The mnemonic WILD and ADDICCTeD can be used to remember the criteria for substance use disorders.

  • W - Work, school, home obligations failure
  • I - InterpersonaL or social consequences
  • D - Dangerous use


  • A - Activities given up or reduced
  • D - Dependence (tolerance)
  • D - Dependence (withdrawal)
  • I - Internal consequences (physical or psychological)
  • C - Can't cut down or control use
  • C - Cravings
  • T - Time-consuming use
  • e
  • D - Duration or amount is greater than intended

The nucleus accumbens is a major input structure of the basal ganglia and integrates cortical and limbic input to mediate goal-directed behaviors.[3] The relative activity of dopamine and serotonin in the nucleus accumbens plays a major part in the development of behavioural addiction and substance use disorders. Dopamine promotes wanting and serotonin satiation. All drugs of addiction have been shown to raise dopamine levels and lower serotonin levels in the NA.

Physiological tolerance is best understood using the example of opioids:

  • When the brain is chronically exposed to elevated levels of opiates, two things develop: (1) opioid tolerance and (2) opioid dependence. Tolerance is the need to take more of the same substance to achieve the same effect, and dependence is being susceptible to opioid withdrawal symptoms. You can only develop withdrawal symptoms if you develop tolerance first.[4]
  • When morphine binds to opiate receptors, it triggers the inhibition of adenylate cyclase, which triggers cytokines to fire impulses. With repeated activation of the opiate receptor by morphine, the enzyme adapts so that morphine no longer cause changes in cell firing. Thus, the effect of a given dose of morphine or heroin is diminished.

Tolerance can also be learned or developed through environmental exposure. This is much like Pavlovian conditioning (Pavlov's Dogs), where environmental stimuli can trigger innate physiological responses. For example, for a heroin user who frequently injects in his bathroom at home, his body will adapt to develop increased tolerance to opiates at that time. If, however, he decided to go rehab, and not use for several weeks, and then injected at a foreign location, his body develop this same learned tolerance, and thus this makes him more susceptible to an overdose.[5] Cases have been reported of patients overdosing due to Pavlovian conditioning.[6]