Prolonged, repeated use of nasal decongestants for symptomatic relief of allergic rhinitis often results in rhinitis medicamentosa (RM), a condition involving "rebound swelling" and additional congestion. Most decongestant sprays contain the preservative benzalkonium chloride (BKC), which causes toxic reactions in the nose, eyes, ears, and lungs, and may exacerbate the symptoms of allergic rhinitis. Recent studies demonstrate the effects of nasal sprays containing BKC or the decongestant oxymetazoline (OXY) in the development of RM. Using rhinostereometry, a technique that measures nasal mucosal swelling and nasal reactivity (with histamine challenge tests), prolonged use of OXY has been shown to induce nasal mucosal swelling and hyperreactivity. Sustained use of BKC alone induces nasal mucosal swelling and, in combination with OXY, BKC appears to have a long-term adverse effect on nasal mucosa. Its presence may also contribute to the RM resulting from overuse of decongestant sprays. Additional research is needed to confirm the deleterious effects of BKC in nasal products. However, these potential effects may be points of clinical differentiation in the treatment of allergic rhinitis and prevention of RM.