Fixed drug eruption (FDE) is a drug reaction manifested by cutaneous and/or mucosal lesions recurring at the same body site each time an individual is re-exposed to the culprit drug. In most cases, FDE develops within minutes to hours of exposure. The oral and anogenital mucosa are the most frequently involved body sites. The oral mucosa may be the only site of involvement or may occur in association with cutaneous and/or genital mucosal lesions.

Lesions begin as sharply demarcated, red or violaceous patches and are often 0.5 cm to several centimeters in size or larger. Occasionally, these lesions will form vesicles or bullae, and on the mucosal skin, they can present as erosions or appear similar to aphthous ulcers. They are often associated with symptoms of burning, but may also be painful or pruritic. The most common presentation is with a solitary lesion, but some individuals may develop multiple lesions. Healing with postinflammatory hyperpigmentation is less commonly seen after oral mucosal involvement.

Numerous drugs have been implicated in causing FDE. Studies have shown certain drugs appear to have a predilection for causing the eruption to occur in or near the mouth. Commonly associated drug classes for FDE in the oral mucosal region are also the most frequently seen culprit medications for FDE in general. These medication classes include antibiotics, in particular sulfonamides, trimethoprim, and tetracyclines, and NSAIDs, including naproxen, ibuprofen, and celecoxib. Other drugs implicated in FDE include amoxicillin, erythromycin, metronidazole, fluconazole, paracetamol (acetaminophen), cetirizine, hydroxyzine, methylphenidate, oral contraceptives, quinine, and phenolphthalein. A nonpigmenting variant is seen with pseudoephedrine.