To estimate the relationship between antiallergy drug prescription rates and suicide across the United States and over time. The relationship between allergy, allergens, and suicidal behavior and suggestions of a possible immune mediation led us to hypothesize that intranasal corticosteroids, known to reduce local airway production of T-helper cell type 2 cytokines, may be associated with reduced risk of suicide relative to antihistamines, which only secondarily affect cytokine production.