Drugs are used for their benefits in treating or curing disease. Although there are inherent risks with using all drugs, 1 in most cases the harm that ensues from drug therapy is mild and brief. An area of concern with many drugs is their propensity to cause allergic reactions, which manifest in many different ways, and can vary in severity. Fortunately, for reasons that are not clear, these allergic reactions affect only a minority of patients, so their perceived harm/benefit balance is still considered to be positive. In this chapter, we provide a general overview of allergic drug reactions, focusing where necessary on those affecting the lung.