BACKGROUND: Allergic rhinitis represents a global health problem. Non-specific nasal hyperresponsiveness is an important feature of allergic and non-allergic rhinitis. This phenomenon is believed to result from the effect of allergic inflammation on the sensory nerves that supply the upper airway mucosa. A pharmacologic agent that has proved useful in the investigation of effects of neuronal stimulation is capsaicin, the pungent component of hot pepper. Intranasal capsaicin specifically stimulates afferent nerves consisting mostly of unmyelinated C fibers and some myelinated A-delta fibers. As a result it can trigger central and axonal reflexes, the latter being putatively mediated by the release of neuropeptides. Capsaicin as a blocking agent of neuropeptides, blocks the axon reflex and may exert a curative effect on allergic rhinitis. OBJECTIVES: To assess the effectiveness of capsaicin for allergic rhinitis in adults. SEARCH STRATEGY: We searched the Cochrane Ear, Nose and Throat Disorders Group Specialised Register, the Cochrane Central Register of Controlled Trials (CENTRAL) (The Cochrane Library Issue 1, 2006), MEDLINE (1966 to 2006) and EMBASE (1974 to 2006). We assessed bibliographies from included studies, and contacted authors of known studies for additional information about published and unpublished trials. The date of the most recent search was January 2006. SELECTION CRITERIA: Randomised controlled trials of capsaicin for allergic rhinitis in adults were included. DATA COLLECTION AND ANALYSIS: Three reviewers read each paper, blind to its identity. Decisions concerning inclusion were made by simple majority. We all performed quality assessment independently. MAIN RESULTS: One small trial did not find evidence that intranasal capsaicin had a therapeutic effect in allergic rhinitis. A small pharmacological effect on clinical histamine dose response was found. After treatment, leukotriene levels in nasal lavage did not increase in the capsaicin group. AUTHORS' CONCLUSIONS: There is insufficient evidence to assess the use of capsaicin in clinical practice.