Preservatives/preservation, ophthalmic chlorhexidine

Although this chapter is directed toward ophthalmic products, it is largely applicable to parenteral and even nonsterile products (solutions, emulsions, and suspensions). The choice of preservative is limited to only a few chemicals that have been found, over the years, to be safe and effective for this purpose. These are benzalkonium chloride, thimerosal, methyl- and propylparaben, phenylethanol, chlorhexidine,... 

Chlorhexidine and its salts are widely used, primarily as topical disinfectants. As excipients, chlorhexidine salts are mainly used as antimicrobial preservatives in ophthalmic formulations. 

Chlorhexidine is also cationic like BKC and exhibits similar incompatibilities. It is not as stable as BKC to autoclaving and may irritate the eyes. It tends to be more favoured in Europe than in the United States, and is particularly used in contact lens products. Chlorobutanol and phenylethyl aclohol are also widely used in ophthalmic products. However, chlorobutanol will hydrolyse in solution, and autoclaving is not usually possible without loss of preservative activity. It is also volatile and may be lost through the walls of plastic containers. 

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