Delivery of drugs into eyes using conventional drug delivery systems, such as solutions, is a considerable challenge to the treatment of ocular diseases. Drug loss from the ocular surface by lachrymal fluid secretion, lachrymal fluid-eye barriers, and blood-ocular barriers are main obstacles. A number of ophthalmic drug delivery carriers have been made to improve the bioavailability and to prolong the residence time of drugs applied topically onto the eye. The potential use of microemulsions as an ocular drug delivery carrier offers several favorable pharmaceutical and biopharmaceutical properties such as their excellent thermodynamic stability, phase transition to liquid-crystal state, very low surface tension, and small droplet size, which may result in improved ocular drug retention, extended duration of action, high ocular absorption, and permeation of loaded drugs. Further, both lipophilic and hydrophilic characteristics are present in microemulsions, so that the loaded drugs can diffuse passively as well get significantly partitioned in the variable lipophilic-hydrophilic corneal barrier. This review will provide an insight into previous studies on microemulsions for ocular delivery of drugs using various nonionic surfactants, cosurfactants, and associated irritation potential on the ocular surface. The reported in vivo experiments have shown a delayed effect of drug incorporated in microemulsion and an increase in the corneal permeation of the drug. 1. Introduction The human eye is a complex structure designed in such a way that its anatomy, physiology, and biochemistry render it almost impervious to foreign agents, including drugs. The human eye has two segments, that is, anterior segment (cornea, conjunctiva, etc.) and posterior segment (vitreous humor, retina, etc.) as shown in detail in Figure 1. The human corneal epithelium represents one of the major rate-limiting barriers which hinders permeation of hydrophilic drugs and macromolecules. Another rate-limiting barrier is stroma which prevents diffusion of highly lipophilic drugs due to abundant hydrated collagen contents [1]. Other significant barriers include lacrimal fluid secretion and lachrymal fluid-eye barriers. Considering these barriers, it is very challenging to develop ocular drug delivery systems which can circumvent these protective barriers and deliver the drug to the posterior segment of the eye without causing permanent tissue damage [2]. Conventional dosage forms like ophthalmic solutions, suspensions, and so forth, are now primordial as they can only deliver the drug to the
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