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Personality and Behavior Changes


Michael B. First

, MD, Columbia University

Last full review/revision Mar 2020| Content last modified Mar 2020
Click here for the Professional Version
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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.

Changes in personality and behavior can be roughly categorized as one of the following:

  • 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 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.

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) or forming words (dysarthria) 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) or to being unable to behave normally. 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. Some types of hallucinations are more likely to be caused by a neurologic disorder. For example, before a seizure occurs, people may smell something when there is no smell (an olfactory hallucination).

Mood extremes

Mood extremes include outbursts of rage, periods of extreme elation (mania) or depression, and, conversely, constant expression of little or no emotion (appearing unresponsive or apathetic).


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

Less commonly, Lyme disease, sarcoidosis, syphilis, or a vitamin deficiency causes personality and behavior changes.


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.

People who have no warning signs should see a doctor within a day or two if the personality or behavior change was recent. If the change occurred gradually over a period of time, people should see a doctor as soon as is practical, but a delay of a week or so is not harmful.

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 with a mental status examination (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).

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 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.

Doctors may examine the interior of the eyes with a handheld device that looks like a small flashlight (called an ophthalmoscope). If doctors see swelling in part of the optic nerve (papilledema), pressure within the skull may be increased, and tumors or bleeding in the brain may be the cause of the mental symptoms.


Some Causes and Features of Personality and Behavior Changes


Common Features*


Mental disorders

Symptoms that occur in episodes lasting a few weeks to a few months and that include mania, depression, or both

Episodes of mania:

  • Elation or irritability

  • Grandiosity

  • Talkativeness

  • Increased activity

  • A decreased need for sleep

  • Racing thoughts, jumping from one idea to another

  • Sometimes hallucinations or delusions of persecution

Episodes of depression:

  • Feeling guilty or worthless

  • Sadness, despair, and a pessimistic mood

  • Loss of interest in typical pleasures

  • Lack of energy

  • Sleeping too much or difficulty sleeping

  • Thoughts of death or suicide

A doctor's examination

Episodes of depression as described in bipolar disorder (see above) without a history of mania

A doctor's examination

Usually symptoms that develop slowly, with mildly disorganized thinking and difficulty coping with daily routines

Later symptoms:

  • Delusions and/or hallucinations

  • Often disinterest and lack of emotion

  • Increasingly disorganized thinking and behavior

  • Difficulty maintaining relationships and employment

A doctor's examination


Use of a drug, particularly

Agitation and sometimes panic or aggression

Sometimes hallucinations

With long-term use of alcohol:

  • Sometimes balance problems

  • Twitching eyes

  • An abnormal way of walking

With long-term use of amphetamines:

  • Sometimes paranoia

With short-term excessive use of amphetamines or cocaine:

  • An increased heart rate

  • Sometimes fever

A doctor's examination

Sometimes blood or urine tests to detect the drug

Sometimes EEG (which records the brain’s electrical activity)

Withdrawal of a drug, particularly

Typically significant confusion and delirium

Shaking (tremors), headache, sweating, fever, and a rapid heart rate or palpitations

Sometimes seizures, hallucinations, and sleep disturbances

Usually in people known to use the drug

A doctor's examination

Sometimes blood or urine tests to check for drugs

Side effects of a drug

Vary, depending on the drug

Drugs with anticholinergic effects, such as

  • Constipation

  • Blurred vision

  • Light-headedness

  • Difficulty starting and stopping urination

  • Dry mouth

A doctor's examination

Sometimes stopping the drug to see whether the symptom goes away

Brain disorders

Symptoms that progress slowly

Loss of short-term memory, difficulty finding the right words, and poor judgment

Difficulty with daily activities (such as balancing a checkbook or finding their way around their neighborhood)

Usually in people over 60

A doctor's examination

Often CT, MRI, or PET of the brain

Detailed testing of mental function involving a series of questions and tasks (neuropsychologic testing)

Brain infections such as


Usually confusion and fever

Pain and/or stiffness when the doctor bends the neck forward (more common in people with meningitis)

With herpes simplex encephalitis, hallucinations of bad odors and sometimes seizures

A spinal tap (lumbar puncture)

Often CT or MRI of the brain

Culture of samples of blood, urine, and material from the throat


Forgetfulness and headaches

Emotional instability in the weeks after a significant head injury

CT or MRI of the brain

Testing of IQ and executive functions such as the ability to plan and solve problems (neurocognitive testing)

Brain tumors or bleeding in the brain (brain hemorrhage)

With brain tumors, a headache that develops gradually and is often worse during the night or early morning and when lying flat

With hemorrhage, a headache that starts suddenly (called a thunderclap headache)

Often confusion and drowsiness

Sometimes seizures

CT or MRI of the brain

Weakness and/or numbness that comes and goes in different parts of the body

Sometimes partial loss of vision or double vision

Sometimes symptoms that are worsened by heat (such as a warm bath or hot weather)

MRI of the brain and spinal cord

Sometimes a spinal tap

Nerve conduction studies (measuring how fast nerves transmit signals) and electromyography (stimulating muscles and recording their electrical activity)

Tremors of the hands and fingers while they are at rest

Stiffness and difficulty moving and maintaining balance

Slowed speech and limited facial expressions

A doctor's examination

Seizure disorders (typically complex partial seizures)

Episodes of abnormal behavior

Usually confusion and staring

Sometimes involuntary chewing, smacking of the lips, and purposeless movements of the limbs

Typically no loss of consciousness and no general shaking of the body (convulsions)

Sometimes hallucinations of odor or taste

MRI of the brain


Symptoms that appear suddenly

Usually weakness or paralysis on one side of the body and unsteadiness when walking

CT or MRI of the brain

Bodywide (systemic) disorders

Hypoglycemia (a low level of blood sugar)

Weakness, sweating, and confusion

Almost always in people taking drugs for diabetes

Tests to measure the blood sugar (glucose) level

Swelling of the legs, loss of appetite, and nausea

Weakness that typically develops over several weeks

Blood and urine tests to evaluate how well the kidneys are functioning

Yellow color of the skin and/or whites of the eyes (jaundice)

Usually swelling of legs and/or abdomen

A reddish purple rash of tiny dots (petechiae)

Usually in people already known to have a liver disorder

Blood tests to evaluate how well the liver is functioning (liver tests)

Usually painful, swollen joints

Often a rash, particularly on the face or areas exposed to sunlight

Sometimes a headache

Blood tests to check for certain antibodies

Thyroid disorders, including

Typically in hyperthyroidism: Palpitations, excessive sweating, difficulty tolerating heat, an increased appetite, weight loss, shakiness (tremor), and sometimes bulging eyes

Typically in hypothyroidism: Fatigue, constipation, difficulty tolerating cold, decreased appetite, weight gain, slow speech, sluggishness, a puffy face, drooping eyelids, coarse and thick dry skin, and loss of eyebrow hair

Blood tests to evaluate how well the thyroid gland is functioning

Vitamin deficiency, such as deficiency of thiamin or vitamin B12

Disorientation, an impaired memory, and irritability

Abnormal sensations in the hands and feet

Other symptoms, depending on which vitamin is deficient

Blood tests to measure vitamin levels

* Features include symptoms and results of the doctor's examination. Features mentioned are typical but not always present.

CT = computed tomography; EEG = electroencephalography; IQ = intelligence quotient; MRI = magnetic resonance imaging; PET = positron emission tomography.


Typically, tests include the following:

  • Measurement of the oxygen level in the blood using a sensor clipped to the person's fingertip (called pulse oximetry)

  • Blood tests to measure sugar (glucose) levels

  • Blood tests to measure levels of alcohol and levels of any antiseizure drugs the person is taking

  • Urine tests to check for drugs

  • A complete blood count (CBC)

  • Sometimes blood tests to measure electrolyte levels and to evaluate kidney function

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). Tests may include

  • Computed tomography (CT) or magnetic resonance imaging (MRI) of the brain: If symptoms of mental dysfunction have just appeared or if people have delirium, a headache, a recent head injury, or any abnormality detected during the neurologic examination

  • A spinal tap (lumbar puncture): If people have symptoms of meningitis or if results of CT are normal in people with a fever, a headache, or delirium

  • Blood tests to evaluate thyroid function: If people are taking lithium, have symptoms of a thyroid disorder, or are over 40 years old and have personality or behavior changes that have just started (particularly people with a family history of thyroid disorders and women)

  • Chest x-ray: If people have a fever or a productive cough or they cough up blood

  • Blood cultures (to check for bacteria in the bloodstream): If people are very ill and have a fever

  • Blood tests to evaluate liver function: If people have symptoms of a liver disorder or a history of alcohol or drug abuse or if specific information about them is not available


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). 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 confusion or delirium, fever, headache, symptoms that suggest brain malfunction, or a recent head injury and about 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.

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