Unusual or strange behavior involves performing actions that are not normal for the person.

Acting strangely

Unusual or strange behavior may include:

• Loss of memory that continues over time or gets worse
• Loss of the ability to concentrate and perform other mental tasks

There are many causes of unusual or strange behavior, including medical and psychiatric illnesses. Two of the more common causes are:

• Delirium -- Sudden or quick onset of reduced consciousness, awareness, perception, or thought that may be a symptom of a medical illness such as brain or mental dysfunction
• Dementia -- Chronic, worsening loss of cognitive function that occurs with brain disorders

Possible causes of strange behavior in older people include:

• Alcohol in excess
• Alzheimer's disease
• Chronic exposure to cold (hypothermia)
• Dehydration
• Electrolyte abnormality
• Emotional problems (depression or feeling useless)
• Head injury (subdural hematoma)
• Heart attack, pulmonary embolism, stroke
• Infection (including pneumonia, gastroenteritis, urinary tract infection)
• Malnutrition (particularly vitamin B12 and thiamine deficiency)
• Medications
• Thyroid disorders (either underactive or overactive)
• Unfamiliar surroundings

Possible causes in people of all ages include:

• Anxiety, emotional problems
• Diseases affecting the nervous system (neurological diseases)
• Illicit drug use (such as amphetamines and cocaine)
• Environmental hazards
• Low or high thyroid function
• Non-neurological diseases, especially those with fever (for example, pneumonia)
• Side effects of antidepressants, such as Prozac
• Side effects of medication for attention deficit disorder, such as Ritalin