A hallucination is a distorted perception of objects or events that involve your five senses: sight, sound, smell, touch, and taste. Hallucinations appear to be real, but they are not. Hallucinations are caused by chemical reactions and/or abnormalities in your brain.

Hallucinations are typically a symptom of a psychotic disorder, particularly schizophrenia, but they can also be caused by substance abuse, neurological conditions, and certain temporary situations.

A person may have a hallucination with or without the realisation that what they're seeing isn't real. A psychotic symptom is when a person believes their hallucination is real.

There are various types of hallucinations, including:

The most common type of hallucination is auditory (sound) hallucination. They entail hearing sounds that aren't real, such as music, footsteps, or doors slamming. Some people hear voices even when no one is speaking. Positive, negative, or neutral voices may be heard. They may order you to do something that will cause you or others harm.
Visual (sight) hallucinations involve seeing things that aren't real, such as objects, shapes, people, animals, or lights.
Tactile (touch) hallucinations: These hallucinations cause you to feel unreal touch or movement on your body. They could include sensations such as bugs crawling on your skin or your internal organs moving around.

Presence hallucinations occur when you have the impression that someone is in the room with you or standing behind you.
Proprioceptive hallucinations occur when you believe your body is moving, such as flying or floating, when it is not.
There are also sleep-related hallucinations, which include:

Hypnopompic hallucinations are hallucinations that occur as you awaken from a deep sleep. Most people consider hypnopompic hallucinations to be normal and unremarkable. They may be more common in people who suffer from specific sleep disorders.

Hypnagogic hallucinations are hallucinations that occur as you fall asleep. They are usually brief, and 86% of them are visual. People frequently see moving patterns and shapes, as well as vivid images of faces, animals, and scenes. These hallucinations are not real.

A person who begins to hallucinate and becomes detached from reality should seek immediate medical attention. Many medical and psychological conditions that can cause hallucinations can quickly escalate into emergencies. The individual should not be left alone.

Contact your doctor, go to the emergency room, or dial 911 or your local emergency number.

A person who detects odours that are not present should be evaluated by a provider as well. Medical conditions such as epilepsy and Parkinson's disease may cause these hallucinations.