The aim of the study is to assess the tolerability, efficacy and safety of a new lithium sulphate prolonged release formulation (Lithiofor®) in patients affected by Bipolar Disorder poor tolerant to lithium immediate-release treatment in terms of lithium-induced tremor when switched from therapy with a lithium carbonate immediate release formulation (Carbolithium®) to a new lithium sulphate prolonged release formulation (Lithiofor®).