We describe a case of anaphylactic reaction in a 46-year-old female post application of decongestant nose drops containing benzalkonium chloride (BAC). With some latency, the patient complained of cough, dyspnea, sensation of heat, croakiness and pruritus. Since she showed all of these symptoms, typical of an anaphylactic reaction, we proceeded some weeks later with a prick test with solutions containing BAC, a cationic surfactant commonly used as an antibacterial preservative in many medical solutions. The prick test was positive, confirming the assumption of a hypersensitive reaction to BAC.