Merck Manual

Please confirm that you are not located inside the Russian Federation

honeypot link



Juebin Huang

, MD, PhD, Department of Neurology, University of Mississippi Medical Center

Reviewed/Revised Feb 2023
Topic Resources

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, and fluctuations in the level of alertness (consciousness).

  • Many disorders, medications, recreational drugs, and poisons cause delirium.

  • Doctors base the diagnosis on symptoms and results of a physical examination, and they use blood, urine, and imaging tests to identify the cause.

  • Promptly correcting or treating the condition causing delirium usually cures it.

Delirium is an abnormal mental state, not a disease. Although the term has a specific medical definition, it is often used to describe any type of confusion.

Overview of Delirium

Delirium is never normal and often indicates a usually serious, newly developed problem, especially in older people. People who have delirium need immediate medical attention. If the cause of delirium is identified and corrected quickly, delirium can usually be cured.

Because delirium is a temporary condition, determining how many people have it is difficult. Delirium affects 15 to 50% of hospitalized people.

Delirium may occur at any age but is more common among older people. Delirium is common among residents of nursing homes. When delirium occurs in younger people, it is usually due to drug use (prescription, over-the-counter, or recreational) or a life-threatening disorder.

What Is Confusion?

Confusion means different things to different people, but doctors use the term to describe people who cannot process information normally.

Confused people cannot

  • Follow a conversation

  • Answer questions appropriately

  • Understand where they are

  • Make critical judgments that affect safety

  • Remember important facts

Confusion has many different causes, including the use of certain drugs (prescription, over-the-counter, and recreational) and a wide variety of disorders. Delirium and dementia, though very different disorders, both cause confusion.

When a person is confused, doctors try to determine what the cause is, particularly whether it is delirium or dementia.

If confusion develops or worsens suddenly, the cause may be delirium. In such cases, medical attention is needed immediately because delirium may be caused by a serious disorder. Also, treating the cause, once identified, can often reverse the delirium.

If confusion develops slowly, the cause may be dementia. Medical attention is needed but not urgently. Treatment may slow the mental decline in people with dementia but usually cannot stop the decline.

Causes of Delirium

Development or worsening of many disorders can cause delirium. Any person can become delirious when extremely ill or taking medications or drugs that affect brain function (psychoactive medications or drugs).

Overall, the most common causes of delirium are the following:

Other causes include hospitalization, surgery, withdrawal of a drug that has been taken for a long time, certain disorders, and poisons. Delirium often develops during hospitalization in people who have dementia.

In some people, no cause can be identified.


In ICUs, people are isolated in a room that typically has no windows or clocks. Thus, people are deprived of normal sensory stimulation and can become disoriented. Sleep is disturbed by staff members who awaken people during the night to monitor and treat them and by loud beeping monitors, intercoms, voices in the hallway, and alarms. Furthermore, most people in ICUs have serious disorders and may be treated with medications that can trigger delirium.

People in ICUs may have seizures Seizure Disorders In seizure disorders, the brain's electrical activity is periodically disturbed, resulting in some degree of temporary brain dysfunction. Many people have unusual sensations just before a seizure... read more that do not cause convulsions (called nonconvulsive seizures). These seizures can cause delirium, but the seizures may not be recognized because they do not cause convulsions or other typical symptoms of seizures. If the seizures are not recognized, they may not be treated appropriately and promptly.


Delirium is also very common after surgery, probably because of the stress of surgery, the anesthetics used during surgery, and the pain relievers (analgesics) used after surgery.

Delirium may also develop when people who are about to have surgery do not have access to a substance they have been using, such as a recreational drug, alcohol, or tobacco. When people stop using such substances, they may have withdrawal Withdrawal Substance use disorders generally involve behavior patterns in which people continue to use a substance (for example, a recreational drug) despite having problems caused by its use. The substances... read more symptoms, including delirium.

Drug use

The most common reversible cause of delirium is use of medications and recreational drugs. In younger people, using recreational drugs and acute intoxication with alcohol Alcohol Use Alcohol (ethanol) is a depressant (it slows down brain and nervous system functioning). Consuming large amounts rapidly or regularly can cause health problems, including organ damage, coma,... read more are common causes. In older people, prescription medications are usually the cause.

Psychoactive drugs directly affect nerve cells in the brain, sometimes causing delirium. They include the following:

Many other drugs can also cause delirium. The following are some examples:

Drug withdrawal

Delirium can also result from suddenly stopping a medication or drug that has been taken for a long time—for example, a sedative (such as a benzodiazepine or barbiturate) or an opioid pain reliever.

Delirium commonly occurs in people who have alcohol use disorder and who suddenly stop drinking alcohol (called delirium tremens Withdrawal symptoms ) and in people who have heroin use disorder and who suddenly stop using heroin.


Abnormal blood levels of electrolytes Overview of Electrolytes More than half of a person's body weight is water. Doctors think about water in the body as being restricted to various spaces, called fluid compartments. The three main compartments are Fluid... read more , such as calcium, sodium, or magnesium, can interfere with the metabolic activity of nerve cells and lead to delirium. Abnormal electrolyte levels may result from use of a diuretic, dehydration, or disorders such as kidney failure and widespread cancer.

If liver failure Liver Failure Liver failure is severe deterioration in liver function. Liver failure is caused by a disorder or substance that damages the liver. Most people have jaundice (yellow skin and eyes), feel tired... read more or kidney failure Overview of Kidney Failure Kidney failure is the inability of the kidneys to adequately filter metabolic waste products from the blood. Kidney failure has many possible causes. Some lead to a rapid decline in kidney function... read more develops and is not diagnosed, a medication that a person has been taking for a long time can cause delirium, even though it previously caused no problems. In these disorders, the liver or kidneys do not process and eliminate medications normally. As a result, medications may accumulate in the blood and reach the brain, causing delirium.

In younger people (once drugs and alcohol are excluded), the cause of delirium is usually

In older people, the cause is often

Such infections can indirectly affect the brain.

Wernicke encephalopathy Wernicke Encephalopathy Wernicke encephalopathy is a brain disorder that causes confusion, eye problems, and loss of balance and results from thiamin deficiency. Wernicke encephalopathy is caused by a severe deficiency... read more , which results from by a severe deficiency of the B vitamin thiamin, can cause confusion and delirium. If untreated, Wernicke encephalopathy can cause severe brain damage, coma, or death.

Delirium may be the first symptom in older people with a viral disease, such as COVID-19.


In younger people, ingestion of poisons, such as rubbing alcohol or antifreeze, is a common cause of delirium.

Spotlight on Aging: Delirium

Delirium is more common among older people. It is a common reason that family members of older people seek help from a doctor or at a hospital. About 15 to 50% of older people experience delirium at some time during a hospital stay.


In older people, delirium can result from any condition that causes delirium in younger people. But it can also result from less severe conditions, such as the following:

Certain age-related changes make older people more susceptible to developing delirium. These changes include

  • An increased sensitivity to medications or drugs

  • Changes in the brain

  • The presence of conditions that increase the risk of delirium

Drugs: Older people are much more sensitive to many medications and drugs. In older people, drugs that affect the way the brain functions, such as sedatives, are the most common cause of delirium. However, drugs that do not normally affect brain function, including many over-the-counter medications (especially antihistamines), can also cause it. Older people are more sensitive to the anticholinergic effects Benefits and Risks of Prescription Drugs Benefits and Risks of Prescription Drugs that many of these medications have. One of these effects is confusion.

Age-related changes in the brain: Delirium occurs more often in older people partly because some age-related changes in the brain make them more susceptible. For example, older people tend to have fewer brain cells and lower levels of acetylcholine—a substance that enables brain cells to communicate with each other. Any stress (due to a drug, disorder, or situation) that causes the level of acetylcholine to further decrease can make it harder for the brain to function. Thus, in older people, such stresses are particularly likely to cause delirium.

Other conditions: Older people are also more likely to have other conditions that make them more susceptible to delirium, such as the following:

  • Stroke

  • Dementia

  • Parkinson disease

  • Other disorders that cause nerve degeneration

  • Use of three or more medications

  • Dehydration

  • Undernutrition

  • Immobility

Delirium is often the first sign of another, sometimes serious disorder. For example, the first symptom in older people with COVID-19 may be delirium, sometimes with no other symptoms of COVID-19.


Delirium tends to last longer in older people.

Confusion, the most obvious symptom, may be harder to recognize in older people. Younger people with delirium may be agitated, but very old people tend to become quiet and withdrawn. In such cases, recognizing delirium is even harder.

Having delirium also increases the risk that older people with COVID-19 will have to stay in an intensive care unit (ICU), go to a rehabilitation facility after being discharged from the hospital, and/or die.

If psychotic behavior develops in older people, it usually indicates delirium or dementia. Psychosis due to a psychiatric disorder rarely begins during old age.

Older people are more likely to have dementia, which makes delirium harder to identify. Both cause confusion. Doctors try to distinguish the two by determining how quickly the confusion developed and what the person’s previous mental function was. Doctors also ask the person a series of questions that test various aspects of thinking (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 ). Doctors usually treat people whose mental function suddenly worsens—even if they have dementia—as if they have delirium until proved otherwise. Having dementia increases the risk of developing delirium, and some people have both.



To help prevent delirium in an older person during a hospital stay, family members can ask hospital staff members to help—by doing the following:

  • Encouraging the person to move around regularly

  • Placing a clock and calendar in the room

  • Minimizing the interruptions and noises during the night

  • Making sure the person eats and drinks enough

Family members can visit and talk with the person and thus help keep the person oriented. People with delirium may be frightened, and the familiar voice of a family member can have a calming effect.

Symptoms of Delirium

Delirium usually begins suddenly and progresses over hours or days. The actions of people with delirium vary but roughly resemble those of a person who is becoming progressively more intoxicated.

The hallmark of delirium is

  • An inability to pay attention

People with delirium cannot concentrate, so they have trouble processing new information and cannot recall recent events. Thus, they do not understand what is happening around them. They become disoriented. Sudden confusion about time and often about place (where they are) may be an early sign of delirium. If delirium is severe, people may not know who they or other people are. Thinking is confused, and people with delirium ramble, sometimes becoming incoherent.

Their level of awareness (consciousness) may fluctuate. That is, people may be overly alert one moment and drowsy and sluggish the next. Other symptoms also often change within minutes and tend to worsen during the evening (a phenomenon called sundowning).

People with delirium often sleep restlessly or reverse their sleep-wake cycle, sleeping during the day and staying awake at night.

People may have bizarre, frightening visual hallucinations, seeing things or people that are not there. Some people develop paranoia (unwarranted feelings of being persecuted) or have delusions (false beliefs usually involving a misinterpretation of perceptions or experiences).

Personality and mood may change. Some people become so quiet and withdrawn that no one notices that they are delirious. Others become irritable, agitated, and restless and may pace. People who develop delirium after taking sedatives are likely to become very drowsy and withdrawn. Those who have taken amphetamines or who have stopped taking sedatives may become aggressive and hyperactive. Some people alternate between the two types of behavior.

Delirium can last hours, days, or even longer, depending on the severity and the cause. If the cause of delirium is not quickly identified and treated, people may become increasingly drowsy and unresponsive, requiring vigorous stimulation to be aroused (a condition called stupor Stupor and Coma Stupor is unresponsiveness from which a person can be aroused only by vigorous, physical stimulation. Coma is unresponsiveness from which a person cannot be aroused and in which the person's... read more ). Stupor may lead to coma or death.

Did You Know...

  • Psychotic behavior that begins during old age usually indicates delirium or dementia.

Diagnosis of Delirium

  • A doctor's evaluation

  • Mental status testing

  • Blood, urine, and imaging tests to check for possible causes

Doctors suspect delirium based on symptoms, particularly when people cannot pay attention and when their ability to pay attention fluctuates from one moment to the next. However, mild delirium may be difficult to recognize. Doctors may not recognize delirium in hospitalized people.

Most people thought to have delirium are hospitalized to evaluate them and protect them from injuring themselves or others. Diagnostic procedures can be done quickly and safely in the hospital, and any disorders detected can be treated quickly.

Because delirium may be caused by a serious disorder (which could be rapidly fatal), doctors try to identify the cause as quickly as possible. Treating the cause, once identified, can often reverse the delirium.

Doctors first try to distinguish delirium from other disorders that affect mental function. Doctors do so by collecting as much information about the person’s medical history as possible, by doing a physical examination, and by testing.

Medical history

Friends, family members, or other observers are asked for information because people with delirium are usually unable to answer. Questions include the following:

  • How the confusion began (suddenly or gradually)

  • How quickly it progressed

  • What the person’s physical and mental health has been like

  • What drugs (including alcohol and recreational drugs, especially if the person is younger) and dietary supplements (including medicinal herbs) the person uses

  • Whether any drugs have been started or stopped recently

Information may also come from medical records, the police, emergency medical personnel, or evidence such as pill bottles and certain documents. Documents such as a checkbook, recent letters, or notification of unpaid bills or missed appointments can indicate a change in mental function.

If delirium is accompanied by agitation and hallucinations, delusions, or paranoia, it must be distinguished from a psychosis due to a psychiatric disorder, such as manic-depressive illness Bipolar Disorder In bipolar disorder (formerly called manic-depressive disorder), episodes of depression alternate with episodes of mania (or a less severe form of mania called hypomania). Mania is characterized... read more or schizophrenia Schizophrenia Schizophrenia is a mental disorder characterized by loss of contact with reality (psychosis), hallucinations (usually, hearing voices), firmly held false beliefs (delusions), abnormal thinking... read more . Typically, people with a psychosis due to a psychiatric disorder do not have confusion or memory loss, and the level of consciousness does not change. Psychotic behavior that begins during old age usually indicates delirium or dementia.


Physical examination

Mental status testing

People who may have delirium are given a mental status test 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 . First, they are asked questions to determine whether the main problem is being unable to pay attention. For example, they are read a short list and asked to repeat it. Doctors must determine whether people take in (register) what is read to them. People with delirium cannot. The mental status test also includes other questions and tasks, such as testing short-term and long-term memory, naming objects, writing sentences, and copying shapes. People with delirium may be too confused, agitated, or withdrawn to respond to this test.


Samples of blood and urine are usually taken and analyzed to check for disorders that doctors think may be causing delirium. For example, abnormalities in electrolyte and blood sugar levels and liver and kidney disorders are common causes of delirium. So doctors usually do blood tests to measure electrolyte and blood sugar levels and to evaluate how well the liver and kidneys are functioning. If doctors suspect a thyroid disorder, tests may be done to evaluate how well the thyroid gland is functioning. Or if doctors suspect that certain medications may be the cause, they may do tests to measure medication levels in the blood. These tests can help determine whether the levels are high enough to have harmful effects and whether a person took an overdose.

Cultures Culture of Microorganisms Infectious diseases are caused by microorganisms, such as bacteria, viruses, fungi, and parasites. Doctors suspect an infection based on the person's symptoms, physical examination results,... read more may be done to look for infections. A chest x-ray may be done to determine whether pneumonia may be the cause of delirium, especially in older people who are breathing fast, whether or not they have a fever or cough.

Computed tomography (CT) or magnetic resonance imaging (MRI) of the brain is usually done.

Electrocardiography (ECG), pulse oximetry (using a sensor that measures oxygen levels in the blood), and a chest x-ray may be used to evaluate how well the heart and lungs are functioning.

In people with a fever or headache, a spinal tap Spinal Tap Spinal Tap (lumbar puncture) may be done to obtain cerebrospinal fluid for analysis. Such analysis helps doctors rule out infection of or bleeding around the brain and spinal cord as possible causes.

Treatment of Delirium

  • Treatment of the cause

  • General measures

  • Measures to manage agitation

Most people who have delirium are hospitalized. However, when the cause of delirium can be corrected readily (for example, when the cause is low blood sugar), people are observed for a short time in the emergency department and can then return home.

Treatment of the cause

Once the cause is identified, it is promptly corrected or treated. For example, doctors treat infections with antibiotics, dehydration with fluids and electrolytes given intravenously, and delirium due to stopping alcohol with benzodiazepines (as well as measures to help people not start drinking alcohol again).

Prompt treatment of the disorder causing delirium usually prevents permanent brain damage and may result in a complete recovery.

Any medications that may be making the delirium worse are stopped if possible.

General measures

General measures are also important.

The environment is kept as quiet and calm as possible. It should be well-lit to enable people to recognize what and who is in their room and where they are. Placing clocks, calendars, and family photographs in the room can help with orientation. At every opportunity, staff and family members should reassure people and remind them of the time and place. Procedures should be explained before and as they are done. People who need glasses or hearing aids should have access to them.

People who have delirium are prone to many problems, including dehydration Dehydration Dehydration is a deficiency of water in the body. Vomiting, diarrhea, excessive sweating, burns, kidney failure, and use of diuretics may cause dehydration. People feel thirsty, and as dehydration... read more , undernutrition Undernutrition Undernutrition is a deficiency of calories or of one or more essential nutrients. Undernutrition may develop because people cannot obtain or prepare food, have a disorder that makes eating or... read more Undernutrition , incontinence Urinary Incontinence in Adults Urinary incontinence is involuntary loss of urine. Incontinence can occur in both men and women at any age, but it is more common among women and older adults, affecting about 30% of older women... read more Urinary Incontinence in Adults , falls Falls in Older Adults A fall is defined as unintentionally or accidentally dropping down to the ground or another lower level. Most falls occur when older adults with one or more physical conditions that impair mobility... read more , and pressure sores Pressure Sores Pressure sores are areas of skin damage resulting from a lack of blood flow due to prolonged pressure. Pressure sores often result from pressure combined with pulling on the skin, friction,... read more Pressure Sores . Preventing such problems requires meticulous care. Thus, people, particularly older people, may benefit from treatment managed by an interdisciplinary team Interdisciplinary care Providing medical care to older people can be complicated. People often have many different health care practitioners at different locations. Travel and transportation issues become more difficult... read more , which includes a doctor, physical and occupational therapists, nurses, and social workers.

Management of agitation

People who are extremely agitated or who have hallucinations may injure themselves or their caregivers. The following measures can help prevent such injuries:

  • Family members are encouraged to stay with the person.

  • The person is put in a room near the nurses’ station.

  • The hospital may provide an attendant to stay with the person.

  • The person's medication regimen is simplified as much as possible.

  • Devices, such as intravenous lines, bladder catheters, or padded restraints, are not used if possible because they can further confuse and upset the person, increasing the risk of injury.

However, sometimes during hospitalization, padded restraints must be used—for example, to keep the person from pulling out intravenous lines and to prevent falls. Restraints are applied carefully by a staff member trained in their use, released at frequent intervals, and stopped as soon as possible because they can upset the person and worsen agitation.

Medications are used to manage agitation only after all other measures have been ineffective. Two types of medications are usually used to control agitation, but neither is ideal:

Doctors are careful when prescribing these medications, particularly for older people. They use the lowest dose possible and stop the medication as soon as possible.

Prognosis for Delirium

Most people with delirium recover fully if the condition causing delirium is rapidly identified and treated. Any delay decreases the chance of a full recovery. Even when delirium is treated, some symptoms may persist for many weeks or months, and improvement may occur slowly. In some people, delirium evolves into chronic brain dysfunction similar to dementia.

Hospitalized people who have delirium are more likely to develop complications in the hospital (including death) than those who do not have delirium. About 35 to 40% of people who have delirium while in a hospital die within 1 year, but the cause of death is often another serious disorder, not delirium itself.

Hospitalized people who have delirium, particularly older people, have a longer hospital stay, higher treatment costs, and a longer recovery time after they leave the hospital.

Drugs Mentioned In This Article

Generic Name Select Brand Names
ARYMO ER, Astramorph PF, Avinza, DepoDur, Duramorph PF, Infumorph, Kadian, MITIGO, MORPHABOND, MS Contin, MSIR, Opium Tincture, Oramorph SR, RMS, Roxanol, Roxanol-T
Demerol, Meperitab
Acid Reducer, Major Acid Reducer, Tagamet, Tagamet HB
Digitek , Lanoxicaps, Lanoxin, Lanoxin Pediatric
INBRIJA, Larodopa
PERSERIS, Risperdal, Risperdal Consta, Risperdal M-Tab, Rykindo, UZEDY
Zyprexa, Zyprexa Intramuscular, Zyprexa Relprevv, Zyprexa Zydis
Seroquel, Seroquel XR
Haldol, Haldol Decanoate
Ativan, Loreev XR
quiz link

Test your knowledge

Take a Quiz!