Merck Manual

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Overview of Substance-Related Disorders


Coreen B. Domingo

, DrPH, Baylor College of Medicine;

Xuefeng Zhang

, Baylor College of Medicine

Last full review/revision Feb 2019| Content last modified Feb 2019
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Drugs are an integral part of everyday life for many people, whether the drugs are used for legitimate medical purposes or recreationally (see table Drugs with Medical and Recreational Uses).

Substance-related disorders can arise when drugs that directly activate the brain's reward system are taken for the feelings of pleasure they induce. The pleasurable sensations vary with the drug. The drugs are divided into 10 different classes based on the different effects they produce in the body:

Substance-related disorders can develop whether or not a drug is legal, is socially acceptable, or has an accepted medical use (with or without a prescription). Details about specific drugs and their effects are discussed elsewhere in The Manual.


Drugs With Medical and Recreational Uses


Medical Use

To treat anxiety and insomnia

To numb surfaces of the body (as a topical anesthetic)

To provide anesthesia

To treat nausea due to advanced cancer, and certain types of childhood seizures

To relieve pain and provide anesthesia

Types of Substance-Related Disorders

Substance-related disorders are usually broken down into two groups:

Substance-induced disorders are problems caused by the direct effects of a drug and include

  • Intoxication

  • Withdrawal

  • Substance-induced mental disorders

Substance use disorders generally involve behavior patterns in which people continue to use a substance despite having problems caused by its use.

Drugs in the 10 classes vary in how likely they are to cause a substance use disorder. The likelihood is termed addiction liability and depends upon a combination of factors including

  • How the drug is used

  • How strongly the drug stimulates the brain's reward pathway

  • How quickly the drug works

  • The drug's ability to induce tolerance and/or symptoms of withdrawal


The terms "addiction," "abuse," and "dependence" have traditionally been used in regard to people with substance use disorders. However, those terms are all too loosely and variably defined to be very useful and also are often used judgmentally. Thus, doctors now prefer to use the more comprehensive and less negative term "substance use disorder."

In discussions about controlled substances and drug use, the term "narcotics" is often used. This term refers to drugs that cause loss of feeling, a sense of numbness, and drowsiness, specifically opioids (drugs that bind to opiate receptors on cells). However, the term "narcotics" is also used in a broader (and inaccurate) sense to include any drug that is illegal or used illegitimately.

Scheduled Drugs

The Comprehensive Drug Abuse Prevention and Control Act (Controlled Substances Act) was implemented in the United States in 1970 and modified over the years to regulate the manufacture and distribution of certain classes of drugs, or controlled substances (see table Some Examples of Controlled Substances*).

Controlled substances are divided into 5 schedules (or classes) based on their potential for abuse, how accepted they are for medical use, and how safe they are when used under medical supervision. Schedule I drugs are considered the most harmful and therefore have the tightest controls. Schedule V drugs are considered the least harmful and are widely available. State and federal schedules may differ (see also US Drug Enforcement Administration Drug Schedules).


Some Examples of Controlled Substances*





Amphetamines, barbiturates (short-acting), cocaine, hydrocodone (including hydrocodone combination products), hydromorphone, methadone, methylphenidate, morphine and other strong opioid agonists, oxycodone, hydrocodone, phencyclidine


Anabolic steroids, barbiturates (intermediate-acting), buprenorphine, dihydrocodeine, dronabinol, ketamine, paregoric


Barbiturates (long-acting), benzodiazepines, chloral hydrate, modafinil, meprobamate, pentazocine, propoxyphene, zolpidem


Cough suppressants containing small amounts of codeine, pregabalin

*The Drug Enforcement Administration maintains a complete alphabetical listing of controlled substances.

Cannot be prescribed.

GHB = gamma hydroxybutyrate; LSD = lysergic acid diethylamide; MDMA = methylenedioxymethamphetamine.

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