Medical and pharmacologic research of recent years has led to the development of many potent and efficacious topical ophthalmic medications. Unfortunately, many of these drugs are potentially toxic or allergenic, and their adverse effects have themselves become important external ocular diseases. This paper presents a classification of these adverse effects and provides a review of their etiology, pathogenesis, histopathology, diagnosis, and management. It is hoped that this information will be helpful to ophthalmologists in their efforts to anticipate, prevent, recognize, and treat drug-induced ocular problems and that it will serve to emphasize the importance of avoiding the ill-considered use of medications.