Causes of Personality & Behaviour Changes


Although individuals sometimes assume that changes in personality, thinking, or behaviour are all due to a mental disorder, there are many possible causes.

All causes ultimately involve the brain, but dividing them into the following four categories can be helpful:

  1. Mental disorders;
  2. Drugs (including drug intoxication, withdrawal, and side effects);
  3. Medical disorders that affect mainly the brain; and
  4. Bodywide (systemic) disorders that also affect the brain.

1. Mental Disorders

Mental disorders include (but not limited to):

  • Bipolar disorder;
  • Depression;
  • Schizophrenia; and
  • Posttraumatic stress disorder.

2. Drugs

Drugs may affect personality or behavior when they cause

  • Intoxication:
    • Particularly alcohol (when consumed in large amounts);
    • Amphetamines;
    • Cocaine;
    • Hallucinogens (such as LSD); and
    • Phencyclidine (PCP).
  • Withdrawal:
    • Alcohol;
    • Barbiturates;
    • Benzodiazepines; and
    • Opioids.
  • Side effects:
    • Drugs intended to affect brain function, such as:
      • Anticonvulsants;
      • Antidepressants;
      • Antipsychotics;
      • Sedatives; and
      • Stimulants.
    • Drugs with anticholinergic effects, such as antihistamines);
    • Opioid pain relievers; and
    • Corticosteroids.

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

3. Medical Disorders that Mainly Affect the Brain

Medical disorders affecting mainly the brain include:

  • Alzheimer disease.
  • Brain infections, such as:
    • Meningitis;
    • Encephalitis; and
    • Human immunodeficiency virus (HIV) infection that involves the brain (called HIV-associated encephalopathy)
  • Brain tumours;
  • Head injuries, such as:
    • A concussion; and
    • Postconcussion syndrome.
  • Multiple sclerosis;
  • Parkinson disease;
  • Seizure disorders; and
  • Stroke.

4. Bodywide Disorders that also Affect the Brain

Bodywide disorders that also affect the brain include:

  • Kidney failure;
  • Liver failure;
  • Low blood sugar (hypoglycemia);
  • Systemic lupus erythematosus (lupus);
  • Thyroid disorders, such as an:
    • Underactive thyroid gland (hypothyroidism); or
    • Overactive thyroid gland (hyperthyroidism).

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