Artificial tears are lubricating eye drops used to relieve dryness and irritation of the ocular surface. Dry eye syndrome (keratoconjunctivitis sicca) is a common ocular surface disorder and is characterized by disruption of the tear film and increased inflammation. The tear film coats the surface of the eye and is composed of 3 layers: an aqueous, lipid, and mucous layer. The aqueous layer consists of a mixture of electrolytes, enzymes, antibodies, antimicrobial proteins, and other compounds. The lipid layer consists of fatty compounds that are responsible for decreasing evaporation of the tear film. The mucous layer contains mucins, gelatinous glycoproteins, which allow for an even distribution of the tear film over the corneal surface and decreased friction during blinking. Artificial tears are used to supplement a patient's naturally-produced tears and increase eye lubrication by mimicking characteristics of the tear film. Artificial tears can be supplemented with other treatments to treat dry eye syndrome and are available over the counter. Artificial tears are also used to moisten contact lenses and in eye examinations. Chemistry Overview Although artificial tears are used to mimic or supplement the roles of the tear film and contain water, electrolytes, and certain polymers, they do not contain the biologically active components found in naturally-produced tears. Moreover, artificial tears often contain chemicals that are not present in naturally-produced tears. Preparations can contain carboxymethyl cellulose, polyvinyl alcohol, hydroxypropyl methylcellulose (a.k.a. HPMC or hypromellose), hydroxypropyl cellulose and hyaluronic acid (a.k.a. hyaluronan, HA). With preservatives Preparations can also contain preservatives in order to prolong usage and avoid bacterial contamination. Preservatives include benzalkonium chloride (BAK), ethylenediaminetetraacetic acid (EDTA), purite, chlorobutanol, sodium perborate, thiomersal, disodium edetate, and oxychloro complex (SOC). Preservatives can be toxic and cause damage to the corneal epithelium. For instance, high concentrations of benzalkonium chloride (BAK) can cause cytotoxicity, cause cell lysis, and reduce tear and mucin production. Patients should limit their use of artificial tears with preservatives to less than four to six times a day. If artificial tears need to be utilized more than four to six times a day, a preservative-free formulation should be used. Preservative-free Preservative-free artificial formulations are indicated for patients who cannot tolerate artificial tear formulations with preservatives, patients who are using artificial tears more than four to six times a day, and patients who are using multiple topical eye drops with preservatives. Preservative-free eye drops are found in single vials. However, single unit-dose