Olfactory dysfunction can be quantitative and/or qualitative. Quantitative smell disorders refer to 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 can’t be measured and refer to disorders in which there is alternation or distortion in the perception of smell. Qualitative disorders include parosmia (also called troposmia) and phantosmia. The term dysosmia refers to a qualitative olfaction disorder and include 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.

Parosmia refers to a distortion in the perception of an odorant. Odorants smell different from what one remembers. A more specific term, cacosmia, refers to an unpleasant perception of an odorant due to nasosinusal or pharyngeal infection.

Phantosmia (phantom smell), also called an olfactory hallucination, is smelling an odor that is not actually there. It can occur in one nostril or both. Unpleasant phantosmia, cacosmia, is more common and is often described as smelling something that is burned, foul, spoiled, or rotten. Experiencing occasional phantom smells is normal and usually goes away on its own in time. When hallucinations of this type do not seem to go away or when they keep coming back, it can be very upsetting and can disrupt an individual's quality of life.

Olfactory hallucinations can be caused by common medical conditions such as nasal infections, nasal polyps, or dental problems. It can result from neurological conditions such as migraines, head injuries, strokes, Parkinson's disease, seizures, or brain tumors. It can also be a symptom of certain mental disorders such as depression, bipolar disorder, intoxication or withdrawal from drugs and alcohol, or psychotic disorders. Environmental exposures are sometimes the cause as well, such as smoking, exposure to certain types of chemicals (e.g., insecticides or solvents), or radiation treatment for head or neck cancer.

A physician can determine if the problem is with the sense of smell (olfactory system) or taste (gustatory system), or if it is caused by a neurological or psychiatric disorder. Phantosmia usually goes away on its own, though this can sometimes be gradual and occur over several years. When caused by an illness (e.g., sinusitis), it should go away when the illness resolves. If the problem persists or causes significant discomfort, a doctor might recommend nasal saline drops, antidepressant or anticonvulsant medications, anesthesia to parts of the nose, or in very rare circumstances, surgical procedures to remove the olfactory nerves or bulbs.

Other olfactory disorders such as hyposmia and anosmia have been found to be a symptom of mood disorders (depression). However, it is not known what olfactory disorders occur and if they are indeed a symptom of a depressive disorder.

It has been found that phantosmia may be an early sign of the neurodegenerative disease Parkinson's disease. It may also be a sign of an intracranial hemorrhage (brain tumours or epilepsy).

Other studies have also found that the symptoms of phantosmia have been alleviated after the patient has been treated for depression.

Another case of a 70-year-old male reported that his first abnormal symptoms were irregular bowel movements. After this the patient developed irregular eye movements and had developed a sleep and behavior disorder after this he developed phantosmia; which was described to be as "stinky and unpleasant". The patient did not display the following symptoms: loss of awareness, confusion, automatisms, convulsive seizures, auditory/visual hallucinations.

Xerostomia is the subjective feeling of oral dryness, which is often (but not always) associated with hypofunction of the salivary glands. The term is derived from the Greek words ξηρός ("xeros") meaning "dry" and στόμα ("stoma") meaning "mouth". A drug or substance that increases the rate of salivary flow is termed a sialogogue.

Hyposalivation is a clinical diagnosis that is made based on the history and examination, but reduced salivary flow rates have been given objective definitions. Salivary gland hypofunction has been defined as any objectively demonstrable reduction in whole and/or individual gland flow rates. An unstimulated whole saliva flow rate in a normal person is 0.3–0.4 ml per minute, and below 0.1 ml per minute is significantly abnormal. A stimulated saliva flow rate less than 0.5 ml per gland in 5 minutes or less than 1 ml per gland in 10 minutes is decreased. The term subjective xerostomia is sometimes used to describe the symptom in the absence of any detectable abnormality or cause. Xerostomia may also result from a change in composition of saliva (from serous to mucous). Salivary gland dysfunction is an umbrella term for the presence of either xerostomia or salivary gland hypofunction.

The differential of hyposalivation significantly overlaps with that of xerostomia. A reduction in saliva production to about 50% of the normal unstimulated level will usually result in the sensation of dry mouth. Altered saliva composition may also be responsible for xerostomia.

The olfactory system is the system related to the sense of smell (olfaction). Many fish activities are dependent on olfaction, such as: mating, discriminating kin, avoiding predators, locating food, contaminant avoidance, imprinting and homing. These activities are referred to as “olfactory-mediated.” Impairment of the olfactory system threatens survival and has been used as an ecologically relevant sub-lethal toxicological endpoint for fish within studies. Olfactory information is received by sensory neurons, like the olfactory nerve, that are in a covered cavity separated from the aquatic environment by mucus. Since they are in almost direct contact with the surrounding environment, these neurons are vulnerable to environmental changes. Fish can detect natural chemical cues in aquatic environments at concentrations as low as parts per billion (ppb) or parts per trillion (ppt).

Studies have shown that exposures to metals, pesticides, or surfactants can disrupt fish olfaction, which can impact their survival and reproductive success. Many studies have indicated copper as a source of olfactory toxicity in fishes, among other common substances. Olfactory toxicity can occur by multiple, complex Modes of Toxic Action.

Early investigation by Hasler and Wisby (1951) examined how fish use olfactory imprinting to discriminate smells in order for fish to find their natal streams. This research provided the framework for testing synthetic chemicals used by hatcheries to examine homing and straying by hatchery fish. The investigation of the toxicity of mercury and copper to the olfactory systems in fish began in the early 1970s. Where they found that solutions of mercury chloride (HgCl) and copper sulfate (CuSO) depressed olfactory response during exposure to the two toxicants and found that toxicant concentration and olfactory response had an inverse relationship to each other.