Lithium, as a very small ion, exerts its action in a complex and mainly intracellular way. It passes not only the neural membrane within the central nervous system (CNS), but also the nonneural membranes of many different cell types. It is therefore not surprising that lithium not only influences mood but also has an impact on other physiological and pathophysiological functions of the organism. It has been used in a broad spectrum of diseases. The evidence for the efficacy of lithium varies considerably depending on the particular medical condition of the patient. (See Table 20.1 for an overview.)

Lithium has been used for different types of headache, with cluster headache being the subtype most widely studied.