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Substance use disorder is classified under Disorders due to substance use in the International Classification of Diseases, 11th Revision (ICD-11), published by the World Health Organization. Substances that can cause addiction are chemical compounds that, once entering the body, produce changes in behavior, mental state, and physical health. These substances are referred to in medical literature as psychoactive substances.
Substance-related disorders are divided into substance use disorders and substance-induced disorders. Substance-induced conditions include intoxication, withdrawal, and substance- or medication-induced mental disorders such as psychotic disorders, bipolar and related disorders, depressive disorders, anxiety disorders, obsessive-compulsive and related disorders, sleep disorders, sexual dysfunctions, delirium, and neurocognitive disorders.
Individuals with substance use disorders may experience various cognitive and behavioral problems. Changes in brain structure and function can lead to intense cravings, personality changes, unusual behaviors, and other behavioral alterations. Brain imaging studies show that substance use causes changes in areas of the brain involved in judgment, decision-making, learning, memory, and behavioral control. Additionally, infants born to dependent women may experience neonatal abstinence syndrome.
People with substance use disorders are also more likely to develop lung or heart disease, stroke, cancer, and various mental health problems. They may be at higher risk of infectious diseases such as HIV and Hepatitis C, as well as premature death.
The severity of physical and mental health problems related to substance use is influenced by factors such as the frequency of use, the amount consumed in a single session, risky behaviors associated with use or context, the method of administration, or a combination of these factors.
Psychoactive Substances and Substance Use Disorders
A psychoactive substance is a chemical compound that affects the central nervous system, altering brain functions and leading to changes in perception, senses, behavior, consciousness, and mood. Certain medications are also classified as psychoactive substances. Even a single use of these substances can result in various disorders related to substance use.
However, addictive substances should not be limited solely to psychoactive compounds. This classification also includes disorders arising from the harmful, non-medical use of substances that are not chemically psychoactive but carry a risk of addiction.
Substance use disorders encompass various conditions resulting from the single or repeated use of psychoactive substances, including certain medications. Additionally, disorders resulting from the harmful, non-medical use of non-psychoactive substances are also included in this category.
Substance use stimulates the brain’s reward pathways. The pleasurable and reinforcing effects of substance use disrupt dopamine regulation in the brain, and this disruption further reinforces repeated use. When substance use becomes repetitive, it can lead to addiction. At the same time, it can cause numerous harms that threaten both mental and physical health.
References
For more detailed information on the topics covered in this section, you may refer to the following sources:
• American Psychiatric Association (2024). What is a Substance Use Disorder? [Link: https://www.psychiatry.org/patients-families/addiction-substance-use-disorders/what-is-a-substance-use-disorder ]
• 101 Questions on Addiction (2019). [Link: 101 Soruda Bağımlılık | Yeşilay (yesilay.org.tr)]
• World Health Organization (2025). ICD-11 for Mortality and Morbidity Statistics, Disorders due to substance use. [Link: https://icd.who.int/browse/2025-01/mms/en#590211325 ]
According to the World Health Organization (WHO), at least two symptoms of substance addiction must occur within the last year, causing significant distress and a decrease in functionality. The symptoms include:
Substance addiction affects various regions of the central nervous system, causing both physical and psychological damage. These effects include:
Some ways for the relatives of substance users include:
Some points to be careful about for the relatives of substance users include:
"No, my child would never use."
• Self-blame and blaming the spouse
"This child turned out this way because of you."
"We couldn't be good parents."
• Feeling disappointment and helplessness
"Did I raise you for this?"
"Everything is over; nothing can be the same again."
• Anger
"I can't have a child like this!"
• Blaming and humiliating the child
"You won't amount to anything."
• Making extreme decisions
"Your school life is over."
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