To present a case of corneal toxicity caused by therapeutic duplication during treatment of acanthamoeba keratitis as a complication of contact lens use.

A 12-year-old girl with a history of wearing orthokeratology contact lenses and a 14-year-old girl with a history of wearing cosmetic contact lenses presented with ocular pain, injection, and decreased visual acuity. They were diagnosed as having acanthamoeba keratitis based on slit lamp examination, confocal microscopy and culture. After the patients were treated with polyhexamethylene biguanide (PHMB) and chlorhexidine, corneal epithelial defect and erosion occurred. Use of chlorhexidine was stopped, and PHMB was used to treat patients and recovery of the corneal epithelium with improvement in symptoms of acanthamoeba keratitis was found.

Using PHMB and chlorhexidine together in treating acanthamoeba keratitis increases the risk of corneal toxicity. Therefore, these drugs should be avoided in combination.