It is possible to achieve a high concentration of many drugs in the eye by applying them as eye drops. In this way, a high local concentration can be reached with minimal risk of systemic side effects. However, the systemic side effects of drops cannot be discounted, particularly in susceptible individuals. For example, timolol drops can precipitate asthma and slow the pulse rate in elderly patients, and pilocarpine drops can cause sweating and nausea. The action of local medications can be prolonged by incorporating them in an ointment, but for most purposes drops are supplied in 5 ml or 10 ml containers. After the container has been opened, it should not be kept for longer than one month. In order to avoid undue stinging, drops can be buffered to near the pH of tears and they contain a preservative, such as benzalconium chloride. It must be borne in mind that patients who develop an allergic reaction to drops might be reacting to the preservative. Single-application containers are also used, which do not contain a preservative but are expensive.
Eye lotions are usually prescribed in 200 ml quantities and are used to irrigate the conjunc-tival sac. Sodium chloride eye lotion is used in first aid to flush out foreign bodies or irritant chemicals. Fresh mains tap water is an adequate substitute.
One of the major drawbacks of using eye drops is that, although high local concentrations of the drug are achieved in the anterior segment of the eye, little if any drug penetrates to the posterior segment. Drops are, therefore, of little use in treating diseases of the vitreous and retina. One way of delivering a drug to the posterior segment is to give it systemically. An example of this is the use of systemic pred-nisolone for posterior uveitis. This treatment method has the drawback of systemic side effects. This can be reduced by delivering the drug to the posterior segment by local injection either directly into the vitreous, along the orbital floor, within the sub-Tenon's space or in the subconjunctival space.
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