Psychiatric symptoms caused by medical illness and drugs

Topic

Depression, anxiety, psychosis or personality change can be the first sign of a body or brain disease or a medicine; finding it changes the treatment.

Updated Nov 13, 2026

Key points

Red flags for a medical cause: onset at an unexpected age (such as new psychosis in an older adult), sudden or fluctuating onset, confusion and inattention, abnormal vital signs and neurological signs.
The type of hallucination is not particularly diagnostic; the whole picture and the tests point to the cause.
Anti-NMDA receptor encephalitis: a young person with new psychosis or catatonia, then seizures, abnormal movements and autonomic instability; look for an ovarian teratoma.
Hypothyroidism can cause depression and memory problems; hyperthyroidism can cause mania and insomnia; Cushing syndrome and corticosteroids can cause depression, mania or psychosis.
Infections to test: HIV (advanced infection can cause cognitive decline and personality change) and syphilis (neurosyphilis can cause psychosis, memory problems, depression or irritability).
Deficiencies: vitamin B12 (irritability, memory problems), thiamin and niacin (memory problems).

What it is

Many medical and neurological conditions, and many medicines and drugs, produce psychiatric symptoms that look like a primary mental disorder. Missing the cause can be fatal (encephalitis, a brain tumour) or leave a treatable illness untreated (hypothyroidism, B12 deficiency). New symptoms, symptoms unlike the person's usual pattern and symptoms unusual for the person's age therefore need a medical history, examination, medication review and basic tests.

Key facts

Delirium is an acute, fluctuating disturbance of attention with a medical cause; confusion and inattention always prompt a medical search.
In autoimmune encephalitis, psychiatric symptoms are often the first sign; seizures, suspected neuroleptic malignant syndrome, abnormal MRI or EEG and inflammatory spinal fluid are red flags.
Poor response or unusual intolerance to antipsychotics (severe rigidity, fever) should prompt a search for a medical cause.
Medicines that can cause psychiatric effects include corticosteroids, anticholinergics (such as antihistamines), interferon and other cancer treatments, and antiseizure medicines.
Withdrawal from alcohol, barbiturates or benzodiazepines, stimulant misuse and other substance use can cause psychiatric symptoms, including psychosis.
Huntington and Parkinson disease can cause depression, mania or psychosis as well as movement problems.

Step by step

Take a timeline: when symptoms began, how fast, and what changed in health, medicines or substance use.
Check vital signs and attention; inattention or fluctuating awareness means delirium until shown otherwise.
Do a neurological examination: focal signs, abnormal movements, pupils, gait.
Review every medicine and substance, including over-the-counter products and recent stops.
Basic tests: blood count, glucose, electrolytes with calcium, kidney and liver tests, thyroid function, B12, HIV and syphilis tests, urine drug screen.
Add head CT or MRI, EEG or lumbar puncture when red flags appear (unexpected age of onset, neurological signs, seizures, catatonia, fever).
Treat the cause first; psychiatric medicines are used for symptoms and are chosen with the medical illness in mind.

Medical causes of psychiatric symptoms, grouped by cause

Group	Example	Typical psychiatric picture	Clue that points to it
Endocrine: thyroid	Hypothyroidism, hyperthyroidism	Depression, memory problems, psychosis; or mania and insomnia	Thyroid function tests
Endocrine: adrenal and steroids	Cushing syndrome, corticosteroid treatment	Depression, mania, personality change, psychosis	Features of Cushing syndrome or recent corticosteroid use
Autoimmune encephalitis	Anti-NMDA receptor encephalitis	Acute psychosis, agitation, catatonia, memory loss	Seizures, dyskinesias, autonomic instability, inflammatory spinal fluid
Systemic autoimmune	Lupus with cerebral arteritis	Confusion, depression, memory problems, psychosis	Other signs of lupus; lupus blood tests
Neurological	Huntington, Parkinson, multiple sclerosis, epilepsy, brain tumour, stroke	Depression, mania, psychosis, personality change	Abnormal neurological examination, seizures
Infection: HIV	Advanced HIV infection	Cognitive decline, memory problems, personality change	Risk history; HIV testing
Infection: neurosyphilis	General paresis	Psychosis, memory and thinking problems, depression or irritability	Treponemal blood tests; spinal fluid testing
Deficiency: B12	Vitamin B12 deficiency	Irritability, memory problems	B12 level
Deficiency: niacin	Pellagra (niacin deficiency)	Memory problems	Poor diet or alcohol use
Metabolic	Liver failure, hypoglycaemia, electrolyte disorders, high calcium	Confusion, psychosis, memory problems	Abnormal blood tests, fluctuating awareness
Drugs and medicines	Anticholinergics, corticosteroids, interferon, antiseizure medicines, stimulants	Confusion, psychosis, depression, mania	Timing with a new medicine or dose change; improvement on stopping it

Test yourself

Think of the answer, then tap the question to see it.

What are four red flags that suggest a medical cause of new psychiatric symptoms?

Onset at an unexpected age (such as new psychosis in an older adult), sudden or fluctuating onset, confusion and inattention, and abnormal vital signs or neurological signs.

A 24-year-old woman develops psychosis, then seizures and abnormal mouth movements. Which diagnosis must be excluded?

Anti-NMDA receptor encephalitis; look for antibodies in spinal fluid and for an ovarian teratoma.

Which medicine group can cause depression, mania or psychosis, like the body's own cortisol excess in Cushing syndrome?

Corticosteroids.

Which two infections should be tested in a new psychiatric presentation with cognitive decline?

HIV and syphilis (neurosyphilis).

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Educational content on behavior and the clinical relationship; not a diagnostic or treatment guide and no substitute for assessment by a health professional. If you or someone near you is at risk, seek help now from an emergency service or a mental-health helpline.

Sources: Red Flags: Clinical Signs for Identifying Autoimmune Encephalitis in Psychiatric Patients (Frontiers in Psychiatry (PubMed Central), 2017); Autoimmune encephalitis: clinical presentation, investigation and treatment (Journal of Neurology (PubMed Central), 2021); Neurosyphilis (MedlinePlus Medical Encyclopedia (U.S. National Library of Medicine), 2024); Medical Assessment of the Patient With Psychiatric Symptoms (MSD Manual Professional Edition, 2026).

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