Which adverse reaction should be monitored in a patient taking ethambutol?

• A. Hepatotoxicity
• B. Optic neuritis
• C. Peripheral neuropathy
• D. Gastrointestinal upset

Answer: (B)

The main concept to recognize is that ethambutol commonly affects the optic nerve, so monitoring vision is essential. Ethambutol can cause optic neuritis, leading to decreased visual acuity and impaired color vision (notably red-green discrimination). Because this side effect is dose- and duration-related, baseline and periodic eye exams with tests of visual acuity and color perception are recommended during therapy. If vision changes occur, pausing or stopping ethambutol is important to prevent permanent damage. While other TB drugs can cause hepatotoxicity, peripheral neuropathy, or GI upset, optic neuritis is the adverse effect most specifically linked to ethambutol.