Editor,—The use of botulinum toxin in the management of ocular motility disorders is well established.1 While transient side effects like ptosis2 and diplopia3 4 due to local spread of the toxin do occur, to our knowledge permanent extraocular muscle damage has not been reported. We report a patient with congenital right superior oblique weakness who underwent botulinum toxin injection to the left inferior rectus muscle. This resulted in permanent and profound loss of inferior rectus muscle function, with atrophy of the muscle, confirmed by magnetic resonance imaging scan.