Mental Illness: Definition, Types, Diagnosis, Treatment

A mental illness is a health condition involving changes in thinking, emotion, or behavior leading to distress or problems functioning in social, work, or family activities. Mental illness is quite common: In 2019, nearly one in five U.S. adults experienced a mental illness, while one in 20 U.S. adults have a serious mental illness. It is estimated that 46% of people who died by suicide had a diagnosed mental health condition and 90% of individuals who died by suicide had shown signs of a mental health condition.

Many people develop a mental illness early on in life, with 50% of all lifetime mental illness beginning by age 14 and 75% by age 24. Its impact on the healthcare system is substantial: Mental illness and substance use disorders are involved in one out of eight emergency room visits.

There are many different types of mental illness—referred to as mental disorders—with different causes, symptoms, and treatments. Some may involve a single episode, while others are relapsing or persistent. To ensure the correct diagnosis and a standardized treatment plan, mental disorders are diagnosed based on criteria outlined in the Diagnostic and Statistical Manual of Mental Disorders, 5th Edition (DSM-5) issued by the American Psychiatric Association.

Sad woman suffering from depression

Ivan Pantic / Getty Images


The DSM-5 categorizes major types of mental disorders by Diagnostic Criteria and Codes.

Anxiety Disorders

Anxiety disorders are the most common mental health concern in the United States, affecting 19.1% of the population. People with anxiety disorders have excessive fear and anxiety and related behavioral disturbances. Their anxiety symptoms can worsen over time, interfering with their ability to function in their daily life. It also leads to attempts to avoid situations or triggers that worsen symptoms.

Anxiety disorders are a group of related conditions, each having unique symptoms. Types of anxiety disorder include:

Bipolar and Related Disorders

Bipolar disorder causes dramatic shifts in a person’s mood, energy, and ability to think clearly. People with this disorder experience extremely high and low moods, known as mania and depression. They may have distinct manic or depressed states, but may also have long periods without symptoms. A person with bipolar disorder can also experience both extremes simultaneously or in rapid sequence. It affects 2.8% of the U.S. population, and 83% of cases are classified as severe.

Bipolar disorders can be categorized into four types, including:

  • Bipolar I disorder 
  • Bipolar II disorder 
  • Cyclothymic disorder or cyclothymia
  • Bipolar disorder, “other specified” and “unspecified”

Depressive Disorders

People with depressive disorders, commonly referred to as simply depression, experience a sad, empty, or irritable mood accompanied by physical and cognitive changes that are severe or persistent enough to interfere with functioning. Some will only experience one depressive episode in their lifetime, but for most, depressive disorder recurs. Without treatment, episodes may last a few months to several years.

Those with depression lose interest or pleasure in activities and have excessive fatigue, appetite changes, sleep disturbances, indecision, and poor concentration. Suicidal thinking or behavior can also occur.

There are many types of depression, including:

  • Major depressive disorder
  • Bipolar depression 
  • Perinatal and postpartum depression
  • Persistent depressive disorder or dysthymia
  • Premenstrual dysphoric disorder
  • Psychotic depression 
  • Seasonal affective disorder (major depressive disorder with seasonal pattern)

Dissociative Disorders 

Dissociative disorders involve problems with memory, identity, emotion, perception, behavior and sense of self. Dissociation refers to a disconnection between a person’s thoughts, memories, feelings, actions or sense of who he or she is. Symptoms of dissociative disorders can potentially disrupt every area of mental functioning.

Examples of dissociative symptoms include the experience of detachment or feeling as if one is outside one’s body and loss of memory, or amnesia. Dissociative disorders are frequently associated with previous experience of trauma. It is believed that dissociation helps a person tolerate what might otherwise be too difficult to bear.

There are three types of dissociative disorders:

  • Dissociative identity disorder
  • Dissociative amnesia
  • Depersonalization/derealization disorder

Feeding and Eating Disorders

People with feeding and eating disorders experience severe disturbances in their eating behaviors and related thoughts and emotions. They become so preoccupied with food and weight issues that they find it harder and harder to focus on other aspects of their life. Over time, these behaviors can significantly impair physical health and psychosocial functioning. Eating disorders affect several million people at any given time, most often women between the ages of 12 and 35. 

There are three main types of eating disorders:

  • Anorexia nervosa 
  • Bulimia nervosa 
  • Binge eating disorder 

Gender Dysphoria

Gender dysphoria refers to psychological distress that results from an incongruence between one’s sex assigned at birth and one’s gender identity. It often begins in childhood, but some people may not experience it until after puberty or much later.

Transgender people are individuals whose sex assigned at birth does not match their gender identity. Some transgender people experience gender dysphoria, and they may or may not change the way they dress or look to align with their felt gender.

Neurocognitive Disorders

Neurocognitive disorders refers to decreased cognitive functioning due to a physical condition. People with this condition may have noticeable memory loss, difficulty communicating, significant problems handling daily tasks, confusion, and personality changes. Neurocognitive disorders can be caused by a wide range of conditions, including Alzheimer’s disease, vascular disease, traumatic brain injury, HIV infection, Parkinson’s disease, and Huntington’s disease.

Types of neurocognitive disorders include:

  • Delirium
  • Major neurocognitive disorder
  • Mild neurocognitive disorder

Neurodevelopmental Disorders 

Neurodevelopmental disorders are a group of disorders in which the development of the central nervous system is disturbed. This can include developmental brain dysfunction, which can manifest as neuropsychiatric problems or impaired motor function, learning, language, or non-verbal communication.

Types of neurodevelopmental disorders include:

Obsessive-Compulsive and Related Disorders

Obsessive-compulsive disorder (OCD) is a disorder in which people have recurring, unwanted thoughts, ideas, or sensations (obsessions) that make them feel driven to do something repetitively (compulsions). These repetitive behaviors can significantly interfere with a person’s daily activities and social interactions. Not performing the behaviors commonly causes great distress. people with OCD have difficulty disengaging from the obsessive thoughts or stopping the compulsive actions. This disorder is estimated to affect 2% to 3% of U.S. adults.

Disorders related to OCD include:

  • Hoarding disorder
  • Body dysmorphic disorder 
  • Body focused repetitive behaviors like excoriation (skin-picking) disorder and trichotillomania (hair-pulling disorder)

Personality Disorders

People with personality disorders have persistent patterns of perceiving, reacting, and relating that are maladaptive and rigid, causing distress and functional impairments. The pattern of experience and behavior begins by late adolescence or early adulthood, and causes distress or problems in functioning. People with personality disorders have trouble dealing with everyday stresses and problems, and they often have stormy relationships with other people.

There are 10 types of personality disorders:

  • Antisocial personality disorder
  • Avoidant personality disorder
  • Borderline personality disorder
  • Dependent personality disorder
  • Histrionic personality disorder
  • Narcissistic personality disorder
  • Obsessive-compulsive personality disorder
  • Paranoid personality disorder
  • Schizoid personality disorder
  • Schizotypal personality disorder

Schizophrenia Spectrum and Other Psychotic Disorders

People with schizophrenia spectrum and other psychotic disorders lose touch with reality and experience a range of extreme symptoms that may include hallucinations, delusions, disorganized thinking and speech, and grossly disorganized or abnormal behavior. Schizophrenia affects less than 1% of the U.S. population.

Other psychotic disorders include:

  • Brief psychotic disorder
  • Delusional disorder 
  • Schizoaffective disorder
  • Substance-induced psychotic disorder

Sleep-Wake Disorders

Sleep-wake disorders, also known as sleep disorders, involve problems with the quality, timing, and amount of sleep, which result in daytime distress and impairment in functioning. They often occur along with medical conditions or other mental disorders, such as depression, anxiety, or cognitive disorders.

There are several types of sleep-wake disorders:

  • Insomnia
  • Obstructive sleep apnea
  • Parasomnias
  • Narcolepsy
  • Restless leg syndrome.

Substance Abuse Disorders

Substance abuse disorders, also called substance use disorder, occurs when a person’s use of alcohol or another substance like drugs leads to health issues or problems at work, school, or home. People with this disorder have an intense focus on using a certain substances to the point where their ability to function in day-to-day life becomes impaired, and they keep using the substance even when they know it is causing or will cause problems.

Trauma-Related Disorders

Trauma-related disorders occur after exposure to a stressful or traumatic event, which can include exposure to physical or emotional violence or pain,abuse, neglect, or catastrophic event. Trauma-related disorders are characterized by a variety of symptoms, including intrusion symptoms (flashbacks),avoidance, changes in mood such as anhedonia (inability to feel pleasure) or dysphoria (dissatisfaction with life), anger, aggression, and dissociation.

Types of trauma-related disorders include:

  • Post-traumatic stress disorder
  • Acute stress disorder
  • Adjustment disorder
  • Reactive attachment disorder
  • Disinhibited social engagement disorder
  • Unclassified and unspecified trauma disorders


Doctors diagnose mental illness by using the criteria outlined in the DSM-5. Many conditions require all criteria to be met before a diagnosis can be made. Others, like borderline personality disorder, require only a set number of criteria from a larger list to be met. 

Many disorders are further classified by severity and specifications that can help doctors determine the appropriate course of treatment for an individual patient. For example, someone being diagnosed with an obsessive-compulsive disorder will also be categorized based upon their level of insight as to whether their OCD beliefs are true and whether or not they present with a current or past history of tic disorder.

About half of people with one mental illness have a comorbid substance use disorder (co-occuring disorder present at the same time or one right after the other). As such, the likelihood of a mental and substance use disorder dual diagnosis is high due to common risk factors and the fact that having one condition predisposes a person to the other. 

It is also common for people to have more than one mental illness at a time. Common comorbidity examples include: 

  • Borderline personality disorder: Other personality disorders, major depression, bipolar disorders, anxiety disorders, and eating disorders
  • Social anxiety disorder: Other anxiety disorders, major depression, and alcohol use disorder
  • Eating disorders: Anxiety, substance use disorder, obsessive compulsive disorder, depression, and post-traumatic stress disorder

It is best for primary care physicians and mental health professionals to work together because a diagnosis as defined by the DSM requires exclusion of other possible causes, including physical causes and other mental disorders with similar features. For example, paranoid delusions can be caused by Huntington’s disease, Parkinson’s disease, stoke, or Alzheimer’s disease, and other forms of dementia.


Due to the wide variety of mental illnesses, many different health professionals may be involved in the treatment process, including:

  • Psychologists
  • Therapists
  • Social workers
  • Psychiatrists
  • Primary care physicians 
  • Pharmacists

Treatment may include one or more of the above professionals and one or more methods (e.g. counseling combined with medication). Treatment-resistant disorders may require further interventions.


Psychotherapy is used to treat a broad range of mental illnesses by helping a person control their symptoms in order to increase functioning, well-being, and healing. 

Common types of psychotherapy include:

  • Cognitive behavioral therapy (CBT): Helps you identify and change maladaptive behaviors
  • Dialectical behavioral therapy (DBT): A form of psychotherapy that uses aspects of CBT along with other strategies including mindfulness that helps you regulate emotions such as those related to suicidal thinking and teaches new skills to change unhealthy and disruptive behaviors
  • Supportive therapy: Helps you build self-esteem while reducing anxiety, strengthening coping mechanisms, and improving social functioning


Medications may be used to reduce symptoms and restore functioning. They are often used in conjunction with psychotherapy.

Four major types of psychotropic drugs include:

  • Antidepressants such as SSRIs, SNRIs, and bupropion are used to treat depression and anxiety, pain, and insomnia. They may also be used to treat ADHD in adults. 
  • Anxiolytics are anti-anxiety medications and are used to treat symptoms ranging from panic attacks to feelings of extreme worry and fear.
  • Antipsychotics are used to treat symptoms of psychosis including delusions and hallucinations. They are often used with other medications to help treat delirium, dementia, and other conditions, including eating disorders, severe depression, and OCD. 
  • Mood stabilizers such as lithium can be used to treat bipolar disorder and mood swings associated with other disorders and also to help with depression. 


Brain stimulation procedures like electroconvulsive therapy (ECT), transcranial magnetic stimulation, and vagus nerve stimulation are used in cases of treatment-resistant and severe depression.

During ECT, electrodes are placed on the head to deliver a series of shocks to the brain to induce brief seizures while the patient is under anesthesia. For transcranial magnetic stimulation, magnets or implants are used to stimulate cells associated with mood regulation.

Ketamine infusion or nasal spray therapy offers another option for people with treatment-resistant major depression. It works rapidly and helps reduce suicide ideation.


Lifestyle changes help promote overall well-being. Healthy lifestyle choices include:

  • Exercising for at least 20 minutes a day
  • Practicing mindfulness in meditation or yoga
  • Avoiding smoking
  • Avoiding substance use (including alcohol)
  • Eating a well-rounded diet that limits fats and refined sugars
  • Having a support system 
  • Maintaining a regular seven- to nine-hour sleep routine
  • Practicing positive thinking

If you are having suicidal thoughts, contact the National Suicide Prevention Lifeline at 1-800-273-8255 for support and assistance from a trained counselor. If you or a loved one are in immediate danger, call 911.

For more mental health resources, see our National Helpline Database

A Word From Verywell

It’s understandable if you’re feeling a little overwhelmed, confused, and even scared when it comes to mental illness. It is possible to live a healthy life with mental illness, and the first step is often the hardest: telling someone about your concerns. Early diagnosis and treatment often improve overall outcome, and talking about it helps further reduce stigma. Remember that there are a lot of ways to manage your mental illness and prevent it from interfering with your daily life.