Ophthalmic products must always be sterile, because the cornea and other transparent parts of the eye are extremely susceptible to irreversible loss of transparency as a result of microbiological infection because they have a particularly poor immune response due to a low blood supply.
Pyrogenicity is of no relevance to ophthalmic products. It is large particles that carry risk of physical damage to the eye. Most eye drops and eye ointments are supplied in multidose, nonresealable containers. Sterility of the contents of these containers is compromised as soon as they are opened. This may be an argument that ophthalmic products are not truly sterile in the same sense that parenteral products are. Nonetheless, they are required to be sterile, and must be manufactured to the same stringent standards of sterility as parenteral products.
Sterility of an ophthalmic product in use is achieved by inclusion of antimicrobial preservatives in their formulations. The inclusion of preservatives is not intended to chemically sterilize the product in manufacture, only to inactivate contaminants that may arise in use.
Typically, these products are allocated two shelf lives. The first, often measured in years, applies to the product while its container is still sealed; the second, usually measurable in weeks, applies after the container is opened. This recognizes the limitations of preservatives used in ophthalmic products in relation to the ability of some microorganisms, given sufficient time, to develop resistance to antimicrobial preservatives.
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