Eosinophils are primarily seen in tissues and are much less abundant in peripheral blood. The clinical significance of eosinophilia is the potential for organ damage with high eosinophil levels.

Major causes are atopic disease, medication allergies, and parasitic infections. Less commonly, primary hematologic disorders cause elevations in eosinophils.

Eosinophilia caused by psychotropic medications is usually mild, transient, and benign. It usually occurs early in treatment. Various mechanisms including a type of hypersensitivity reaction has been proposed. Tricyclic antidepressants , some typical antipsychotics and clozapine cause mild eosinophilia [1, 2]. The eosinophilia rarely causes eosinophilic end organ damage. There are case reports of clozapine-related eosinophilic colitis, pleural effusion, and pancreatitis. Eosinophilia is often a feature of clozapine-induced myocarditis. There are also reports of eosinophilic pulmonary complications from serotonin reuptake inhibitors [3]. Eosinophilia can also be seen as part of a syndrome of Drug Reaction with Eosinophila and Systemic Syndromes (DRESS) . Features of DRESS syndrome are described in Chapter 33.