What Is a Substance Use Disorder?



Substance use disorder (SUD) is complex a condition in which there is uncontrolled use of a substance despite harmful consequence. People with SUD have an intense focus on using a certain substance(s) such as alcohol, tobacco, or illicit drugs, to the point where the person’s ability to function in day to day life becomes impaired. People keep using the substance even when they know it is causing or will cause problems. The most severe SUDs are sometimes called addictions.


People can develop an addiction to:

  • Alcohol

  • Marijuana

  • PCP, LSD and other hallucinogens

  • Inhalants, such as, paint thinners and glue

  • Opioid pain killers, such as codeine and oxycodone, heroin

  • Sedatives, hypnotics and anxiolytics (medicines for anxiety such as tranquilizers)

  • Cocaine, methamphetamine and other stimulants

  • Tobacco

People with a substance use disorder may have distorted thinking and behaviours. Changes in the brain’s structure and function are what cause people to have intense cravings, changes in personality, abnormal movements, and other behaviours. Brain imaging studies show changes in the areas of the brain that relate to judgment, decision making, learning, memory, and behavioural control.


Repeated substance use can cause changes in how the brain functions. These changes can last long after the immediate effects of the substance wears off, or in other words, after the period of intoxication. Intoxication is the intense pleasure, euphoria, calm, increased perception and sense, and other feelings that are caused by the substance. Intoxication symptoms are different for each substance.


When someone has a substance use disorder, they usually build up a tolerance to the substance, meaning they need larger amounts to feel the effects.


According to the National Institute on Drug Abuse, people begin taking drugs for a variety of reasons, including:


  • to feel good — feeling of pleasure, “high” or "intoxication"

  • to feel better — relieve stress, forget problems, or feel numb

  • to do better — improve performance or thinking

  • curiosity and peer pressure or experimenting


In addition to substances, people can also develop addiction to behaviours, such as gambling (gambling disorder).


People with substance use and behavioural addictions may be aware of their problem but not be able to stop even if they want and try to. The addiction may cause physical and psychological problems as well as interpersonal problems such as with family members and friends or at work. Alcohol and drug use is one of the leading causes of preventable illnesses and premature death nationwide.


Symptoms of substance use disorder are grouped into four categories:


  1. Impaired control: a craving or strong urge to use the substance; desire or failed attempts to cut down or control substance use

  2. Social problems: substance use causes failure to complete major tasks at work, school or home; social, work or leisure activities are given up or cut back because of substance use

  3. Risky use: substance is used in risky settings; continued use despite known problems

  4. Drug effects: tolerance (need for larger amounts to get the same effect); withdrawal symptoms (different for each substance)


Many people experience substance use disorder along with another psychiatric disorder. Oftentimes another psychiatric disorder precedes substance use disorder, or the use of a substance may trigger or worsen another psychiatric dis

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