A hallucination is a sensory perception occurring without any external stimulus. This distinguishes it from an illusion, which is a distortion of a sensory perception. Hallucinations can occur in any sensory modality and may present in isolation or as part of a larger clinical problem (particularly an acute confusional state). A hallucination is often a very frightening experience for the sufferer.

Published: 2nd August 2022 | Updated: 15th August 2022

Differential diagnosis

Common Diagnoses

  • Drugs (Such as Amphetamine, Cocaine, LSD, Ecstasy, Solvents and Tricyclic Overdose) and Drug Withdrawal
  • Extreme Fatigue
  • Alcoholic Hallucinosis (Delirium Tremens of Acute Alcohol Withdrawal)
  • Febrile Delirium
  • Schizophrenia

Occasional Diagnoses

  • Severe Metabolic Disturbance of any Cause
  • Temporal Lobe Epilepsy
  • Cerebral Space-Occupying Lesion
  • Psychotic Depression
  • Bereavement Reaction
  • Hypoxia

Rare Diagnoses

  • Narcolepsy
  • Mania
  • Post-Concussional State
  • Iatrogenic: Idiosyncratic Adverse Drug Reaction
  • Near-Death Experience

Ready reckoner

Key distinguishing features of the most common diagnoses

DrugsFatigueAlcohol WithdrawalFebrile DeliriumSchizophrenia
Sudden OnsetYesPossibleYesYesNo
Mainly Auditory HallucinationNoNoPossibleNoYes
Cognition ImpairedYesPossibleYesYesNo

Possible investigations

  • The GP’s use of investigations will depend on the clinical situation. If hallucinations are part of an acute confusional state, particularly in adults, admission is likely to be required and will result in a battery of tests to check, for example, for sources of fever, hypoxia and metabolic disturbance. The following are investigations the GP might use in patients who do not require admission or who are not presenting acutely.
  • Urinalysis: Very useful in the acute situation, particularly in the elderly. May suggest UTI or hyperglycaemic ketotic state or severe dehydration.
  • Pulse oximeter: To detect hypoxia.
  • Glucometer blood glucose: In a known diabetic or if any glycosuria.
  • FBC and LFT: Raised MCV and abnormal LFT suggest chronic alcohol excess.
  • U&E: May reveal electrolyte disturbance as underlying cause.
  • EEG: May suggest diagnosis of temporal lobe epilepsy or narcolepsy.
  • CT scan: The definitive test for a cerebral space-occupying lesion.

Top Tips

  • Delirium in children with a fever is quite common, especially at night and is not in itself a sinister sign; assess possible causes of the fever in the usual way, and if the cause is not serious, reassure the parents as they may be quite frightened by the child’s hallucinations.
  • Patients with anxiety, personality disorder and borderline mental illness may sometimes complain of auditory hallucinations, occasionally because experience has told them that this generates action from health professionals. Genuine auditory hallucinations are usually distressing and often in the second person (psychotic depression) or third person (schizophrenia) – and are accompanied by other hard evidence of mental illness.
  • Minor and transient auditory and visual hallucinations are normal in the recently bereaved – but the patient will need reassurance that he or she isn’t ‘going mad’.

Red Flags

  • Hallucinations caused by drugs, or by drug and alcohol withdrawal, can be terrifying and dangerous for the patient and carers, so admission is likely to be required.
  • Genuine auditory hallucinations strongly suggest psychotic illness, particularly schizophrenia and depression; visual hallucinations are almost always organic in nature.
  • Purely olfactory hallucinations are pathognomic of temporal lobe pathology and require urgent investigation.
  • Tactile hallucinations are very suggestive of acute alcohol withdrawal and occasionally cocaine abuse.
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Website disclaimer

Pulse Reference is based on the best-selling book Symptom Sorter.

The experts behind Pulse Reference are Dr Keith Hopcroft who is the co-author of Symptom Sorter, a GP in Essex and Pulse’s editorial advisor and Dr Poppy Freeman, a GP in Camden and also a clinical advisor to Pulse.

This website is for clinical guidance only and cannot give definitive diagnostic information. Practitioners should work within the limits of their individual professional practice, seek guidance when necessary and refer appropriately.