Latent tuberculosis infection is a state of sustained immune response to Mycobacterium tuberculosis antigen stimulation without evidence of clin-ically evident active tuberculosis (TB). Although there is no symptom of active TB, preventive treatment is recommended because there is a risk of conversion to active disease. The first treatment option in latent tuberculosis prophylaxis is isoniazid (INH). We presented a child who developed sleepiness, weakness and aggressive mood while taking isoniazid (INH) prophylaxis for latent TB in our clinic. The clinician should be aware that there may be serious side effects of INH, including the central nervous system. Patients should be evaluated for drug side effects during fol-low-up and the patient should be informed.