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Olfactory hallucinations are a type of hallucination which involves the olfactory system. Patients who experience olfactory hallucinations think that they are smelling something when there is nothing in the environment producing the odor being experienced. They may be able to describe the smell in detail, and they can have reactions to the smell, such as gagging at the smell of feces or increased saliva production at the smell of chocolate chip cookies. Although phantosmia, as it is known to the medical community, may seem primarily like an interesting curiosity, it can actually be a sign that a patient has a severe medical problem, and people who smell things which don't exist definitely need to see a doctor.
One of the most common causes for these hallucinations is brain damage. The olfactory system is a physical sensory system, just like the systems used to allow people to perceive touch and vision, and damage to that system can interfere with its function. People with tumors or severe head injuries can start to smell phantom odors as a result of confused neurons along the sensory system's pathways.
Phantosmia can also be a symptom of epilepsy, caused by temporal lobe seizures which trigger the brain into thinking that a smell is present. Some people with migraines have also described hallucinations related to smell during the aura phase before a migraine sets in. Other causes of olfactory hallucinations include exposure to certain toxins, some types of drugs, and physical damage to the olfactory system, such as an infection.
Some psychiatric conditions have also been linked with phantom odors. People with severe psychiatric disorders may have profound chemical imbalances in their brains which trigger the chemosensory system which allows people to perceive smell, creating a hallucinatory experience.
The smells experienced can be good or bad, with more people tending to report strong or unpleasant odors. Olfactory hallucinations should not be confused with parosmia, in which a smell is not processed correctly by the brain, causing the smell to change in the perception of a nose's owner. In an example of parosmia, someone might smell a rose and complain that it smells like rust, smoke, tar, or something else. In this case, a smell is present and being perceived, but it's not being perceived properly.
People who experience olfactory hallucinations should consult a neurologist or psychiatrist who can determine the cause and make treatment recommendations. Diagnosis may include the use of medical imaging to look at the brain and the olfactory system for signs of abnormalities.