Phantosmia is an olfactory hallucination. A hallucination is the sensory perception of something with no basis in reality, as opposed to an olfactory illusion, which is a misinterpretation of a physical stimulus; in the case of an odor it is known as parosmia. Phantosmia is the perception of a smell in the complete absence of any physical odor. The perceived odor can range from pleasant to disgusting. Although the causes of phantosmia are uncertain, it often occurs with neurological and psychological disorders such as schizophrenia, mood disorders, Parkinsons disease, epilepsy, neuroblastoma, and frequent migraines.Galen also mentioned olfactory hallucinations in his work and stated that these hallucinations constitute the signs of an oncoming disease.Different types of phantosmia include: Unirhinal (single nostril), episodic, and recurrent, where the activation of brain's GABAergic system seems to play a role in the inhibition of the unirhinal phantosmia.Treatments for phantosmia range from drug therapies (e.g., venlafaxine) and brain stimulation therapies to invasive surgical procedures involving removal of the olfactory bulbs or olfactory epithelium.The word phantosmia is a noun of Greek origin. It is composed of two words: (1) phant- meaning ""phantom"" and (2) -osmia (from osme) meaning ""smell"".