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Quaternary Ammonium Compounds

Benzalkonium chloride is a quaternary ammonium compound and is used as a preservative in many ophthalmic solutions. Typically in dilutions of 0.01% or less. It is and has been a "popular" preservative, and it's use in solutions for many years is probably due to the well documented reports of the toxic effects and allergy problems.
"Several authors have demonstrated cytoxic effects of BAC on the cornea, even with percentages as low as 0.005%( Burstein. Invest Ophthal Vis Sci 1980;19:308). A study matching unpreserved against 0.02% BAC solutions in methylcellulose found little corneal epithelial damage despite exagerated use.( Berdy. Arch Ophth 1992;110:528)"

Alkyl triethanol ammonium chloride, benzethonium chloride, CETRIMIDE (cetyltrimethylammonium bromide), DYMED and POLYQUAD are also examples of quaternary ammonium compounds that are used in ophthalmic preparations.

The quatenary ammonium compounds have a surfactant quality which has a detergent like property, disrupting cell membranes, and they are also bacteriostatic or bacteriocidal depending on the concentrations used.