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Personality and Behavior Changes


Michael B. First

, MD, Columbia University

Reviewed/Revised Apr 2022 | Modified Sep 2022
Topic Resources

Healthy people differ significantly in their overall personality, mood, and behavior. Each person also varies from day to day, depending on the circumstances. However, a sudden, major change in personality and/or behavior, particularly one that is not related to an obvious event (such as taking a drug or losing a loved one), often indicates a problem.

Sudden changes in personality and behavior can be roughly categorized as involving one of the following types of symptoms:

  • Confusion or delirium

  • Delusions

  • Disorganized speech or behavior

  • Hallucinations

  • Mood extremes (such as depression or mania)

These categories are not disorders. They are just one way doctors organize different types of abnormal thought, speech, and behavior. These changes in personality and behavior can be caused by physical or mental health problems.

People may have more than one type of change. For example, people with confusion due to a drug interaction sometimes have hallucinations, and people with mood extremes may have delusions.

Confusion and delirium

Confusion and delirium Delirium Delirium is a sudden, fluctuating, and usually reversible disturbance of mental function. It is characterized by an inability to pay attention, disorientation, an inability to think clearly... read more refer to a disturbance of consciousness. That is, people are less aware of their environment and, depending on the cause, may be excessively agitated and belligerent or drowsy and sluggish. Some people alternate between being less alert and being overly alert. Their thinking appears cloudy and slow or inappropriate. They have trouble focusing on simple questions and are slow to respond. Speech may be slurred. Often, people do not know what day it is, and they may not be able to say where they are. Some cannot give their name.

Delirium often results from a serious, newly developed physical problem or a reaction to a drug, especially in older people. People who have delirium need immediate medical attention. If the cause of delirium is identified and corrected quickly, delirium often resolves.


Delusions are fixed false beliefs that people hold despite evidence to the contrary. Some delusions are based on a misinterpretation of actual perceptions and experiences. For example, people may feel persecuted, thinking that a person behind them on the street is following them or that an ordinary accident is purposeful sabotage. Other people think that song lyrics or newspaper articles contain messages that refer specifically to them (called a delusion of reference).

Some beliefs seem more plausible and can be difficult to identify as delusions because they could occur or have occurred in real life. For example, people occasionally are followed by government investigators or have their work sabotaged by coworkers. In such cases, a belief can be identified as a delusion by how strongly people hold the belief despite evidence to the contrary.

Other delusions are easier to identify. For example, in religious or grandiose delusions, people may believe they are Jesus or the president of the country. Some delusions are quite bizarre. For example, people may think that their organs have all been replaced by machine parts or that their head contains a radio that receives messages from the government.

Disorganized speech

Disorganized speech refers to speech that does not contain the expected logical connections between thoughts or between questions and answers. For example, people may jump from one topic to another without ever finishing a thought. The topics may be slightly related or entirely unrelated. In other cases, people respond to simple questions with long, rambling answers, full of irrelevant detail. Answers may be illogical or completely incoherent. This type of speech differs from the difficulty expressing or understanding language (aphasia Aphasia Aphasia is partial or complete loss of the ability to express or understand spoken or written language. It results from damage to the areas of the brain that control language. People may have... read more ) or forming words (dysarthria Dysarthria Dysarthria is loss of the ability to articulate words normally. Speech may be jerky, staccato, breathy, irregular, imprecise, or monotonous, but people can understand language and use it correctly... read more ) that is caused by a brain disorder such as stroke.

Occasionally misspeaking or intentionally being evasive, rude, or humorous is not considered disorganized speech.

Disorganized behavior

Disorganized behavior refers to doing quite unusual things (such as undressing or masturbating in public or shouting and swearing for no apparent reason). People with disorganized behavior typically have trouble doing normal daily activities (such as maintaining good personal hygiene or obtaining food).


Hallucination refers to hearing, seeing, smelling, tasting, or feeling things that are not actually there. That is, people perceive things, seemingly through their senses, that are not caused by an outside stimulus. Any sense can be involved. The most common hallucinations involve hearing things (auditory hallucinations), usually voices. The voices often make derogatory comments about the person or command the person to do something.

Not all hallucinations are caused by a mental disorder. Psychedelic drugs Hallucinogens Hallucinogens are a class of drugs that cause profound distortions in people's perceptions. Hallucinogens distort and intensify sensations, but the actual effects are variable and unpredictable... read more , such as LSD, mescaline, and psilocybin, are referred to as hallucinogens because they can cause visual hallucinations. Some types of hallucinations are more likely to be caused by a neurologic disorder Introduction to Symptoms of Brain, Spinal Cord, and Nerve Disorders Disorders that affect the brain, spinal cord, and nerves are called neurologic disorders. Neurologic symptoms—symptoms caused by a disorder that affects part or all of the nervous system—can... read more . For example, before a seizure occurs, people may smell something when there is no smell (an olfactory hallucination).

Mood extremes


Although people sometimes assume that changes in personality, thinking, or behavior are all due to a mental disorder, there are many possible causes. All causes ultimately involve the brain, but dividing them into four categories can be helpful:

  • Mental disorders

  • Drugs (including drug intoxication, withdrawal, and side effects)

  • Disorders that affect mainly the brain

  • Bodywide (systemic) disorders that also affect the brain

Mental disorders

Mental disorders include


Drugs may affect personality or behavior when they cause

Rarely, certain antibiotics and drugs used to treat high blood pressure cause changes in personality and behavior.

Disorders that mainly affect the brain

These disorders can affect personality, mood, and behavior. They include

Bodywide disorders that also affect the brain

Bodywide disorders that also affect the brain include


During the initial evaluation, doctors try to determine whether symptoms are due to a mental or physical disorder.

The following information can help people decide when a doctor’s evaluation is needed and help them know what to expect during the evaluation.

Warning signs

In people with changes in personality or behavior, certain symptoms and characteristics are cause for concern. These warning signs include

  • Symptoms that appear suddenly

  • Attempts to harm themselves or others or threats to do so

  • Confusion or delirium

  • Fever

  • Severe headache

  • Symptoms that suggest malfunction of the brain, such as difficulty walking, balancing, or speaking or vision problems

  • A recent head injury (within several weeks)

When to see a doctor

People who have warning signs should be seen by a health care practitioner as soon as possible. Law enforcement may need to be called if people are violent.

What the doctor does

Doctors first ask questions about the person's symptoms and medical history. Doctors then do a physical examination, including a neurologic examination Neurologic Examination When a neurologic disorder is suspected, doctors usually evaluate all of the body systems during the physical examination, but they focus on the different parts of the nervous system. Examination... read more with a mental status examination Mental Status When a neurologic disorder is suspected, doctors usually evaluate all of the body systems during the physical examination, but they focus on the different parts of the nervous system. Examination... read more (which evaluates the ability to pay attention, memory, mood, and ability to think abstractly, follow commands, and use language, among other things). What they find during the history and physical examination often suggests a possible cause of the changes and the tests that may need to be done (see table Some Causes and Features of Personality and Behavior Changes Some Causes and Features of Personality and Behavior Changes Some Causes and Features of Personality and Behavior Changes ).

Questions include when symptoms began. Many mental disorders begin in a person's teens or 20s. If a mental disorder begins during middle age or later, especially if there is no obvious trigger (such as loss of a loved one), the cause is more likely to be a physical disorder. A physical disorder is also more likely to be the cause when mental symptoms change significantly during middle age or later in people with a chronic mental disorder. If changes began recently and suddenly in people of any age, doctors ask about conditions that can trigger such changes. For example, they ask whether people have just started or stopped taking a prescription or recreational drug.

Doctors ask about other symptoms that may suggest a cause, such as

  • Palpitations: Possibly an overactive thyroid gland or use or withdrawal of a drug

  • Tremors: Parkinson disease or withdrawal of a drug

  • Difficulty walking or speaking: Multiple sclerosis, Parkinson disease, stroke, or intoxication from an opioid or a sedative

  • Headache: Brain infection, brain tumor, or bleeding in the brain (hemorrhage)

  • Numbness or tingling: A stroke, multiple sclerosis, or a vitamin deficiency

People are also asked whether they have previously been diagnosed and treated for a mental or seizure disorder. If they have been treated, doctors ask whether they have stopped taking their drugs or decreased the dose. However, because people with mental disorders may also develop physical disorders, doctors do not automatically assume that any new abnormal behavior is caused by the mental disorder.

Doctors ask about physical disorders people have (such as diabetes) and about their lifestyle (such as their marital status, job situation, educational background, use of alcohol, tobacco, and recreational drugs, and living arrangements). Doctors also ask whether family members have had any physical disorders that can cause mental symptoms (such as multiple sclerosis).

During the physical examination, doctors look for signs of physical disorders that can cause changes in mental status, particularly the following:

  • Fever and/or a rapid heart rate (suggesting an infection, alcohol withdrawal, or use of amphetamines or cocaine in high doses)

  • Confusion or delirium (suggesting drug intoxication or withdrawal)

  • Abnormalities during the neurologic examination, such as difficulty forming words or understanding language (possibly suggesting a brain disorder)

Confusion and delirium are more likely to result from a physical disorder. People with mental disorders are rarely confused or delirious. However, many physical disorders that cause changes in behavior do not cause confusion or delirium, but they often cause other symptoms that may appear to be a mental disorder.

Doctors bend the person's neck forward. If doing so is difficult or painful, meningitis may be the cause. Doctors check the legs and abdomen for swelling, which may result from kidney or liver failure. If the skin or whites of the eyes look yellow, the cause may be liver failure.



Typically, tests include the following:

For most people known to have a mental disorder, no further testing is needed if their only symptoms are worsening of their typical symptoms, if they are awake and alert, and if results of these tests and their physical examination are normal.

For most other people, blood tests to check for HIV infection are usually done.

Other tests are done based mainly on the symptoms and examination results (see table Some Causes and Features of Personality and Behavior Changes Some Causes and Features of Personality and Behavior Changes Some Causes and Features of Personality and Behavior Changes ). Tests may include


The underlying condition is corrected or treated when possible. Whatever the cause, people who are a danger to themselves or others typically need to be hospitalized and treated whether they are willing or not. Many states require that such decisions be made by someone appointed to make health care decisions for the mentally ill person (called a surrogate decision maker Default Surrogate Decision Making If a person is unable to make decisions about personal health care, some other person or people must provide direction in decision making. The general term for such person is surrogate decision... read more ). If the person has not appointed a decision maker, doctors may contact the next of kin, or a court may appoint an emergency guardian.

People who are not dangerous to themselves or others can refuse evaluation and treatment, despite the difficulties their refusal may create for themselves and their family.

Key Points

  • Not all changes in personality and behavior are due to mental disorders.

  • Other causes include drugs (including withdrawal and side effects), disorders that mainly affect the brain, and bodywide disorders that affect the brain.

  • Doctors are particularly concerned about people with symptoms that suggest brain malfunction, such confusion or delirium, fever, headache, people with a recent head injury and people who want to harm themselves or others.

  • Typically, doctors do blood tests to measure the levels of oxygen, sugar (glucose), and certain drugs (such as antiseizure drugs) the person is taking, and they may do other tests based on the symptoms and results of the examination.

Drugs Mentioned In This Article

Generic Name Select Brand Names
Eskalith, Eskalith CR, Lithobid
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