An Addiction Science Network Resource

Drug Classification

This section describes several common schemes for classifying drugs with psychoactive properties. For common street names of illicit drugs, see Illicit Drug Index. For a brief commentary regarding the commonly used distinction between “hard” and “soft” drugs, see Hard and Soft Drugs.

Chemical Structure

Compounds are often classified according to their chemical structures. Although this is useful for medicinal chemists, it does not provide a meaningful classification scheme for categorizing drug effects. Some compounds with similar chemical structures produce very similar biological effects (e.g., morphine, heroin), but others which belong to the same chemical class often produce much different effects (e.g., apomorphine, nalorphine). Furthermore, compounds which differ in chemical structure often produce similar biological effects (e.g., amphetamine, cocaine).

Pharmacological Activity

This scheme classifies drugs according to their primary pharmacological activity. All compounds produced multiple effects, so what is consider the primary effect and what is considered the secondary effect varies as a function of reference point. Often the primary therapeutic use of a compound is considered its primary effect and thus used to classify it pharmacologically.

The classification scheme given below focuses on each compound's main psychotropic effects which in some cases classifies it differently from what might be considered its primary pharmacological effect (i.e., based on therapeutic use). For example, pseudoephedrine is a popular decongestant that has mild stimulatory properties. Pseudoephedrine's  decongestant effect might be considered its primary effect, while its stimulatory effect would be considered a secondary side-effect. However, from a psychopharmacological perspective, pseudoephedrine's stimulatory effect is its primary effect and its decongestant action is a secondary (although therapeutically more useful) effect. Therefore, pseudoephedrine is classified below as a mild stimulant like caffeine and nicotine.

Drug Class

Primary Effects/Approved Medicinal Uses


Opiates/Opioids/Narcotic Analgesics

analgesia, cough suppression, antidiarrhea, suppression of opiate withdrawal, sedation; currently used therapeutically for the first four effects

opium, morphine, codeine, heroin  (diacetyl morphine), fentanyl, methadone, meperidine, L-alpha-acetylmethadol (LAAM)

Narcotic/Opiate Antagonists

block the effects of narcotics; used to treat opiate overdose

naloxone, naltrexone

Psychomotor Stimulants

stimulate psychological and sensory-motor functioning; used therapeutically to treat ADHD and narcolepsy, sometimes as an appetite suppressant, occasionally antifatigue, formerly for asthma and for sinsus decongestion

amphetamine, methamphetamine, cocaine, methylphenidate

Other Stimulants

similar to psychomotor stimulants but with much less efficacy; various therapeutic effects including caffeine compounded with aspirin in some OTC pain relievers, ephedrine in OTC asthma medicines, pseudoephedrine in OTC sinus decongestants and OTC appetite suppressants 

caffeine, nicotine, ephedrine, pseudoephedrine


general decrease in CNS arousal/excitability level; used therapeutically for anesthetic, anticonvulsant, sedative, and hypnotic effects

thiopental, secobarbital, pentobarbital, phenobarbital

Minor Tranquilizers

general decrease in CNS arousal/excitability level, but low dose are somewhat selective for anxiety and much less sedative than barbiturates; used therapeutically as anxiolytics, benzodiazepines also as anesthetics and anticonvulsants

includes two subclasses: benzodiazepines (e.g.,. diazepam, chlordiazepoxide, flunitrazepam [Rohypnol]) and muscle relaxants (e.g., meprobamate)

Major Tranquilizers (antipsychotics/neuroleptics)

general sedation at high doses, with selective antipsychotic activity at lower doses; used therapeutically to treat schizophrenia and other major psychotic disorders

haloperidol, pimozide, flupenthixol, chlorpromazine, spiroperidol, clozapine


no perceptible CNS effects in normals, but effectively alleviate depression in many depressives; used therapeutically to treat depression

includes three subclasses: monoamine oxidase inhibitors (e.g., pargyline), tricyclic antidepressants (e.g., amitriptyline, desmethylimipramine), and selective serotonin reuptake inhibitors (SSRIs: e.g., sertaline)


dampens extreme mood swings in some people; used to treat manic-depressive (bipolar) disorders



general decrease in CNS arousal/excitability level; no current therapeutic uses, but formerly used as an anesthetic and a sedative

ethyl alcohol (other alcohols have similar actions but are associated with very toxic effects, e.g., methanol)

Volatile Anesthetics

general decrease in CNS arousal/excitability level; used therapeutically for anesthesia

nitrous oxide, halothane, ether

Volatile Solvents

produce feelings of intoxication, can produce hallucinations at high doses; no therapeutics uses (all can cause marked brain damage in moderately low concentrations

toluene, benzene, naphtha


produce altered states of consciousness; hallucinogenics produce hallucinations sometimes reported as "mystic" experiences; cannabinoids usually produce increased feelings of "well being" and "mellow" intoxication; the "pleasantness" of the states produced by both classes probably depends partially on expectancies; no approved therapeutic uses, but cannabinoids are being increasingly used for their antinausea, anxiolytic, and appetite-stimulating effects in severely ill patients (e.g., AIDS) 

includes two subclasses: hallucinogenics (e.g., lysergic acid diethylaminde [LSD], mescaline, psilocybin) and cannabinoids (e.g., marijuana, hashish).

Stimulatory Hallucinogenics (cf. former psychotomimetics)

produce a mixture of psychomotor stimulant and hallucinogenic effects, depending on dose and other factors; no therapeutic uses, except phencyclidine as a veterinary anesthetic

MDMA (ecstasy), phencyclidine (PCP), ketamine (?)

Abbreviations: ADHD, attention deficit hyperactivity disorder; AIDS, acquired immune deficiency syndrome; CNS, central nervous system; OTC, over-the-counter (nonprescription) medicines.

DEA Drug Schedules

The Controlled Substances Act of 1970 divided substances to be regulated into 5 schedules. These schedules govern the legal distribution and use of most substances with a significant abuse liability. (For details of the Controlled Substances Act and its revisions, see Appendix C, Agents Manual--DEA SENSITIVE.) The Drug Enforcement Administration  is the primary federal agency charged with enforcing these regulations and with coordinating national and international efforts to reduce illicit drug supply. Hence, this classification scheme is often refereed to as the DEA Schedules.




Medical Use



Schedule I



use only

  • marijuana, THC
  • LSD, mescaline, peyote
  • heroin

Schedule II



with no refills

  • amphetamine, methamphetamine, cocaine
  • codeine, levorphanol, meperidine
    methadone, morphine, opium
  • amobarbital, pentobarbital, secobarbital
  • phencyclidine

Schedule III

Moderately High


written or
with refills

  • Tylenol with codeine, paregoric
  • chlorphentermine
  • anabolic steroids

Schedule IV



written or
with refills

  • chloral hydrate
  • chlordiazepoxide, diazepam, flunitrazepam
  • meprobamate
  • methohexital, phenobarbital

Schedule V



not necessary

Robitussin A-C (contains less than 100 mg codeine per 100 ml)

Note: Drugs are continually being reclassified. The above listing (except for Schedule V) is from the information contained on the DEA's license application/renewal form (DEA-225) dated April 1988. 

Addiction Liability

It is virtually impossible to reach consensual agreement regarding the relative addiction liability of most drugs. However, scientific agreement is generally good regarding the drugs that produce the strongest indications of addiction. These drugs belong to two pharmacological classes—psychomotor stimulants and opiates—and can be considered the prototypical addictive drugs. Agreement is also good regarding the relatively low addiction liability of some over-the-counter medicines, such as aspirin, diphenhydramine (an antihistamine), and pseudoephedrine (a decongestant). Some commonly used substances have controversial addiction liabilities (e.g., caffeine, nicotine).

Want more information? Search our website:


ASNet Home

ASNet Profile

Illicit Drug Index

Virtual Lab Tour

Research Reports

Drug Classification

A Primer on Drug Addiction

Experimental Methods

Treatment Resources

Biological Basis of Addiction

Before Prohibition: Early Psychoactive Medicines

Links to Other Websites

Click here to enter the Addiction Science Network Discussion Forum

Brain Reward System© 1999-2009 Addiction Science Network
This page was last revised 20 March 2009 18:20 EDT.
Send comments to:
Report technical problems to: