Dysosmia is a disorder described as any qualitative alteration or distortion of the perception of smell. Qualitative alterations differ from quantitative alterations, which include anosmia and hyposmia. Dysosmia can be classified as either parosmia (also called troposmia) or phantosmia. Parosmia is a distortion in the perception of an odorant. Odorants smell different from what one remembers. Phantosmia is the perception of an odor when no odorant is present. The cause of dysosmia still remains a theory. It is typically considered a neurological disorder and clinical associations with the disorder have been made. Most cases are described as idiopathic and the main antecedents related to parosmia are URTIs, head trauma, and nasal and paranasal sinus disease. Dysosmia tends to go away on its own but there are options for treatment for patients that want immediate relief.

Smell disorders can result in the inability to detect environmental dangers such as gas leaks, toxins, or smoke. In addition to safety, nutritional and eating habits can also be affected. There is a loss of appetite because of unpleasant flavor and fear of failing to recognize and consuming spoiled food. A decreased or distorted sense of smell therefore results in a decreased quality of life. Distortions are believed to have a greater negative impact on people than the complete loss of smell because they are constantly reminded of the disorder and the distortions have a greater effect on eating habits.

Olfactory dysfunction can be quantitative and/or qualitative. Quantitative smell disorders are disorders in which there is complete or partial loss of olfaction. Anosmia, the complete loss of olfaction, and hyposmia, the partial loss of olfaction are the two disorders classified as quantitative because they can be measured. Qualitative smell disorders cannot be measured and are disorders in which there is alternation or distortion in the perception of smell. Qualitative disorders include parosmia (also called troposmia) and phantosmia. Dysosmia is a qualitative olfaction disorder and includes both parosmia and phantosmia. Olfactory dysfunction including anosmia, hyposmia, and dysosmia can be either bilateral or unilateral on either nostril. Anosmia only on the left nostril would be termed unilateral left anosmia while bilateral anosmia would be termed total anosmia. If the distortion is unpleasant, the disorder can be referred to as cacosmia. Under an alternative definition, cacosmia is used for an unpleasant perception of an odorant due specifically to nasosinusal or pharyngeal infection. The rare term torquosmia can be used when the perceived smell is chemical, burning or metallic.

Parosmia is a distortion in the perception of an odorant. Odorants smell different from what one remembers.

Phantosmia is the perception of an odor when there is no odorant present. When a phantom smell lasts less than a few seconds, the term olfactory hallucination can be used.^{[citation needed]}

Even though the causes of dysosmia are not yet clear, there are two general theories that describe the etiology: the peripheral and central theories. In parosmia, the peripheral theory refers to the inability to form a complete picture of an odorant due to the loss of functioning olfactory receptor neurons. The central theory refers to integrative centers in the brain forming a distorted odor. In phantosmia, the peripheral theory refers to neurons emitting abnormal signals to the brain or the loss of inhibitory cells that are normally present in normal functioning. The central theory for phantosmia is described as an area of hyper-functioning brain cells that generate the order perception. Evidence to support these theories include findings that for the majority of individuals with distortions, there is a loss of sensitivity to smell that accompanies it and the distortions are worse at the time of the decreased sensitivity. It has been reported in parosmia cases that patients can identify triggering stimuli. Common triggers include gasoline, tobacco, coffee, perfume, fruits and chocolate.

The cause of dysosmia has not been determined but there have been clinical associations with the neurological disorder:

Most of cases are described as idiopathic and the main antecedents related to parosmia are URTIs, head trauma, and nasal and paranasal sinus disease. Psychiatric causes for smell distortion can exist in schizophrenia, alcoholic psychosis, depression, and olfactory reference syndrome.