Microbial contamination of ophthalmic drops means a risk of serious injury to the eye. Ophthalmic drops must therefore comply with sterility requirements. Protection of multiple-dose drops against secondary contamination is ensured by an addition of an antimicrobial agent. Selection of a suitable antimicrobial agent is conditioned by many factors, such as the spectrum of effect, properties of the preparation, compatibility with the components of the preparation and the container, and the technology of manufacture. Although the added antimicrobial substance ensures the safety of the preparation, on the other hand it can produce a number of negative effects in the eye tissue. The present paper summarizes pharmacopoeial requirements for microbial quality of ophthalmic drops, outlining the properties and efficacy of antimicrobial substancescommonly used in ophthalmic drops (benzalkonium chloride BAC, cetrimide CTM, phenyl mercuric salts PHg, thiomersalTM, chlorobutanol ChB, benzyl alcohol BA, phenyl ethyl alcohol PEA, chlorohexidin ChX, parabens PB), their typical concentrations and combinations, including the parameters of formulation and the interactions which affect their activity. It deals with the toxicity of these antimicrobial substances, side effects on the eye tissue, and alternatives to the use of antimicrobial agents.

        Key words: ophthalmic drops – antimicrobial substances – toxicity – eye tolerance