Abstract: Provided is a method of treating patients with Cluster Headache (CH) including chronic Cluster Headache (cCH) comprising intranasal administration to a patient in need thereof of an aqueous composition, e.g., pharmaceutical composition, comprising a therapeutically effective amount of ketamine, which may reduce or eliminate side effects of ketamine when used in the treatment of Cluster Headache, and to a composition, e.g., pharmaceutical composition, comprising a therapeutically effective amount of ketamine for use in the treatment of Cluster Headache, wherein the composition is in a nasal dosage form such as a nasal spray.