Merck Manual

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Substance Use 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
Click here for the Professional Version
Click here for the Professional Version

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

The substances involved tend to be members of the 10 classes of drug that typically cause substance-related disorders:

These substances all directly activate the brain's reward system and produce feelings of pleasure. The activation may be so strong that people intensely crave the substance. They may neglect normal activities to obtain and use the drug.

Substance use 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.

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."

Recreational and illicit substance use

Use of illegal drugs, although problematic from a legal standpoint, does not always involve a substance use disorder. On the other hand, legal substances, such as alcohol and prescription drugs (and marijuana in an increasing number of states in the United States), may be involved in a substance use disorder. Problems caused by use of prescription and illegal drugs cut across all socioeconomic groups.

Recreational drug use has existed in one form or another for centuries. People have used drugs for a variety of reasons, including

  • To alter or enhance mood

  • As part of religious ceremonies

  • To gain spiritual enlightenment

  • To enhance performance

People who take drugs recreationally may take them occasionally in relatively small doses, often without doing themselves harm. That is, users do not develop drug withdrawal, and the drug does not physically harm them (at least in the short term). Drugs usually considered recreational include opium, alcohol, nicotine, marijuana, caffeine, hallucinogenic mushrooms (see also Mushroom (Toadstool) Poisoning), and cocaine. Many recreational drugs are considered "natural" because they are close to their plant origin. They contain a mixture of low-concentration psychoactive ingredients rather than isolated psychoactive compounds.

Recreational drugs are usually taken by mouth or inhaled.


People usually progress from experimentation to occasional use and then to heavy use and sometimes to a substance use disorder. This progression is complex and only partially understood. The process depends on interactions between the drug, user, and setting.


Drugs in the 10 classes vary in how likely they are to cause a substance use disorder. The likelihood is termed addiction liability. Addiction liability 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

In addition, substances that are legally and/or readily available, such as alcohol and tobacco, are more likely to be used first. As people continue to use a substance, they often see less risk in using it and may begin to increase their use and/or experiment with other substances. People's perception of risk also may be influenced by the social and legal consequences of use.

During treatment of medical illness or following surgical or dental procedures, people are routinely prescribed opioids. If people do not take the whole amount prescribed, the drugs sometimes end up in the hands of people who wish to use them recreationally. Because the use of these drugs for nonmedical purposes has become such a large problem, many health care providers have responded by

  • Prescribing lower amounts of opioid drugs

  • Encouraging people to safely store or dispose of any leftover drugs

  • Expanding prescription take-back programs


Factors in users that may predispose to a substance use disorder include

  • Physical characteristics

  • Personal characteristics

  • Circumstances and disorders

Physical characteristics likely include genetic factors, although researchers have yet to find more than a few biochemical and/or metabolic differences between people who develop a substance use disorder and those who do not.

People with low levels of self-control (impulsivity) or high levels of risk-taking and novelty-seeking behaviors may have an increased risk of developing a substance use disorder. However, there is little scientific evidence to support the concept of the addictive personality that has been described by some behavioral scientists.

A number of circumstances and coexisting disorders appear to increase the risk of a substance use disorder. For example,

  • People who are sad, emotionally distressed, or socially alienated may find temporary relief from drug use, which can lead to increased use and sometimes to a substance use disorder.

  • People with other, unrelated mental disorders such as anxiety or depression are at increased risk of developing a substance use disorder. (Doctors use the term "dual diagnosis" to refer to people who have both a mental disorder and a substance use disorder.)

  • People with chronic pain often require opioid drugs for relief. Some of these people later develop a substance use disorder.

However, in many of these patients, nonopioid drugs and other treatments do not adequately relieve pain and suffering.


Cultural and social factors are very important in initiating and maintaining (or relapsing to) substance use. Watching family members (eg, parents, older siblings) and peers using substances increases the risk that people will begin using substances. Peers are a particularly powerful influence among adolescents (see Substance Use and Abuse in Adolescents). People who are trying to stop using a substance find it much more difficult if they are around others who also use that substance.

Doctors may inadvertently contribute to harmful use of psychoactive drugs by overzealously prescribing them to relieve stress. Many social factors, including mass media, contribute to patients' expectation that drugs should be used to relieve all distress.


  • A doctor's evaluation

  • Sometimes a person's self-report

Sometimes a substance use disorder is diagnosed when people go to a health care practitioner because they want help stopping use of a drug. Other people try to hide their drug use, and doctors may suspect problems with drug use only when they notice changes in a person's mood or behavior. Sometimes doctors discover signs of substance use during a physical examination. For example, they may discover track marks caused by repeatedly injecting drugs intravenously. Track marks are lines of tiny, dark dots (needle punctures) surrounded by an area of darkened or discolored skin. Injecting drugs under the skin causes circular scars or ulcers. People may claim other reasons for the marks, such as frequent blood donations, bug bites, or other injuries.

Health care practitioners also use other methods (such as questionnaires) to identify a substance use disorder. Urine and sometimes blood tests may be done to check for the presence of drugs.

Criteria for diagnosis

The criteria for diagnosing a substance use disorder fall into four categories:

  • The person cannot control use of the substance.

  • The person's ability to meet social obligations is compromised by use of the substance.

  • The person uses the substance in physically dangerous situations.

  • The person shows physical signs of use and/or dependence.

Inability to control use

  • The person takes the substance in larger amounts or for a longer time than originally planned.

  • The person desires to stop or cut down use of the substance.

  • The person spends a lot of time obtaining, using, or recovering from the effects of the substance.

  • The person craves the substance.

Social impairment

  • The person fails to fulfill major role obligations at work, school, or home.

  • The person continues to use the substance even though it causes (or worsens) social or interpersonal problems.

  • The person gives up or reduces important social, occupational, or recreational activity because of substance use.

Risky use

  • The person uses the substance in physically hazardous situations (eg, when driving or in dangerous social circumstances).

  • The person continues to use the substance despite knowing it is worsening a medical or psychologic problem.

Physical symptoms*

  • Tolerance: The person needs to use increasingly more of the substance to feel the desired effect.

  • Withdrawal: Unpleasant physical effects occur when the substance is stopped or when it is counteracted by another substance.

*Note that some drugs, particularly opioids, sedative/hypnotics, and stimulants, can result in tolerance and/or withdrawal symptoms even when taken as prescribed for legitimate medical reasons and for relatively brief periods (less than 1 week for opioids). Withdrawal symptoms that develop following appropriate medical use do not warrant the diagnosis of a substance use disorder. For example, when people with severe pain due to advanced cancer become dependent (psychologically and physically) on an opioid such as morphine, their withdrawal symptoms are not considered evidence of a substance use disorder.

People who have 2 or more of these criteria within a 12-month period are considered to have a substance use disorder. The severity of the substance use disorder is determined by the number of criteria met:

  • Mild: 2 to 3 criteria

  • Moderate: 4 to 5 criteria

  • Severe: ≥ 6 criteria


  • Varies depending on substance and circumstances



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