Pharmacology for Addiction

Drug Classification

Drugs can be classified three ways: behavioral classification, pharmacodynamic classification, and legal classification.

Behavioral classification includes five main categories: stimulants, opioids, sedative hypnotics, antipsychotics, antidepressants, and psychedelics (Table 2.1). Each of these categories is more or less self-explanatory.

  • Stimulants include drugs that stimulate or produce arousal and behavioral activation. Examples of stimulants are cocaine, amphetamines, nicotine, and caffeine.
  • Opioids are natural, semisynthetic, or synthetic drugs that bind to opioid receptors and produce analgesia. Analgesia can be defined as the reduction of pain or elevation of pain thresholds.
  • Sedative hypnotics are drugs that sedate or decrease arousal, producing an anti-anxiety effect, hypnosis, or sleep. Hypnosis is defined as the induction of sleep. Two examples of this class of drugs are alcohol and benzodiazepines.
  • Antipsychotics are drugs that are used to treat psychosis and include the classic antipsychotics such as haloperidol (trade name: Haldol), and modern second generation drugs, such as olanzapine (trade name: Zyprexa).
  • Antidepressants are drugs that are used to treat major depressive episodes and include selective serotonin reuptake inhibitors, such as fluoxetine (trade name: Prozac) and escitalopram (trade name: Lexapro), among others.
  • Psychedelics are drugs that produce psychedelic experiences. Psychedelic can be defined as mind-altering. Another term that is often used to describe this drug class is hallucinogen, but the true meaning of the term hallucination is to experience something that is not there; therefore, the term psychedelic is preferred. Psychedelics include lysergic acid diethylamide (LSD) and psilocybin (derived from psychedelic mushrooms).


Drug Class







Meperidine (Demerol)



Diazepam (Valium)






Fluoxetine (Prozac)



Lysergic acid diethylamide


A pharmacodynamic classification can utilize the same broad behavioral categories mentioned above and adopt them to describe the pharmacodynamic effects on brain neurotransmission (Table 2.1). For example, stimulants are indirect dopamine agonists. Opioids are direct opioid receptor agonists. Sedative hypnotics directly or indirectly facilitate γ-aminobutyric acid neurotransmission. Antipsychotics are currently dopamine D2 receptor antagonists and serotonin 5-HT2 receptor antagonists. Antidepressants are serotonin reuptake inhibitors, norepinephrine reuptake inhibitors, or a combination of serotonin/norepinephrine reuptake inhibitors. Psychedelics all facilitate serotonergic activity either directly or indirectly by increasing serotonin release. Legal classification involves two categories: prescription vs. nonprescription and drug abuse. The modern era of the legal classification of drugs with abuse potential in the United States began in 1970 with the passage of the Controlled Substances Act. Five Schedules were created. The Department of Justice (Drug Enforcement Administration and Federal Bureau of Investigation) and Department of Health and Human Services (Food and Drug Administration) determine which drugs are on which schedule. The classification decisions are made on the basis of specific criteria for the potential of abuse, accepted medical use in the United States, and the potential for dependence. Drugs are classified on a continuum of increasing abuse potential, with or without a medical use.

Schedule I:

No officially recognized medical use, lack of accepted safety for use under medical supervision, high abuse potential, and cannot be legally prescribed in the United States. Examples: heroin, LSD, _9-THC, and methylenedioxymethamphetamine (MDMA or Ecstasy).

Schedule II:

Officially recognized medical use and high abuse potential that may lead to severe psychological or physical dependence. Examples: methamphetamine, morphine.

Schedule III:

Officially recognized medical use and abuse potential that is less than Schedules I and II that can lead to moderate or low physical dependence or high psychological dependence. Examples: ketamine, drug products that contain less than 15 mg hydrocodone per unit (Vicodin).

Schedule IV:

Officially recognized medical use and low abuse potential compared with Schedule III. Examples: alprazolam (Xanax), diazepam (Valium).

Schedule V:

Officially recognized medical use and low abuse potential relative to Schedule IV. These drugs consist mainly of preparations that contain limited amounts of certain narcotics. Example: Robitussin AC cough syrup, which contains less than 200 mg codeine per 100 ml.

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