To report a case involving an unexpected increase in intraocular pressure (IOP) and acute angle closure after oral administration of methazolamide.

A 38-year-old male visited the emergency department complaining of decreased visual acuity (VA) and ocular pain. These symptoms developed after he took two tablets of 50 mg methazolamide because his IOP was above normal after a short course of systemic steroid treatment. His uncorrected VA dropped to 0.04 and the refractive error was −6.5 diopters in both eyes. The anterior chamber was very shallow, and the IOPs were 46 mmHg in the right eye and 42 mmHg in the left eye. Macular retinal folds were observed in both eyes in infrared fundus images. The patient was instructed not to take methazolamide, which was suspected as the cause of this idiosyncratic drug reaction. He was prescribed topical anti-glaucoma medications and cycloplegics to relieve the acute angle closure, and all symptoms disappeared after these treatments.

Methazolamide is a sulfa derivative like topiramate, which can cause acute angle closure involving edema of the ciliary body and anterior displacement of the lens-iris diaphragm. Clinicians should consider this possible IOP increase before prescribing methazolamide.