Benzalkonium chloride (BAK) is a widely used quaternary ammonium cationic detergent. It has occasionally been reported as a cause of allergic dermatitis. The agent is the most commonly used preservative in ophthalmic medications, solutions for contact lenses, and "artificial tears."
A physician developed a severe allergic conjunctivitis from an ophthalmic solution containing BAK. The conjunctivitis became worse with the use of another preparation containing this preservative. The patch test reaction to BAK was strongly positive. The only other instance of allergic conjunctivitis from BAK, confirmed with a positive patch test, was published in 1952.
French investigators report that allergic hypersensitivity to BAK may produce systemic reactions from the administration of chemically related antihypertensive and antispasmodic compounds.
Alexander A. Fisher, Michael A. Stillman. Allergic Contact Sensitivity to Benzalkonium ChlorideCutaneous, Ophthalmic, and General Medical Implications. Arch Dermatol. 1972;106(2):169–171. doi:10.1001/archderm.1972.01620110005001