SALT Study (Switching to the Administration of Lithium Slow -Release Treatment)

Inclusion Criteria: 1. Male or female patients aged 18 to 65 years (limits included), with no limitation of race. Female patients with childbearing potential should have a negative pregnancy test and should not be breastfeeding. Male and female patients should use an appropriate birth control method. 2. Diagnosis of Bipolar disorder (BD) I or II (as per DSM-5), with or without rapid cycling. 3. BD patients in treatment with lithium Carbolithium® immediate-release, presenting at the screening and baseline low tolerability in terms of tremor (tremor severity assessment ≥ 2 on single item of the UKU side-effect rating scale). 4. MADRS score ≤ 10 and YMRS ≤ 12 at screening and baseline visits. 5. Patients legally capable to give the consent to participate in the study and available to sign and date the written informed consent. 6. Patient able to understand the study procedures and to comply with protocol requirements. Exclusion Criteria: 1. Fulfilling criteria for the following disorders: schizophrenia, psychotic and schizoaffective disorders, unipolar depression; concomitant organic mental disorder or intellectual disability; history of dementia or cognitive disorders, any neurodegenerative diseases. 2. Known hypersensitivity or allergy to lithium or to any components of the study medications. 3. Pharmacological treatments affecting tremor, except for patients treated for at least 2 months before the screening visit (i.e. Beta-blockers; for further details). 4. Known tremor due to irreversible lithium neurotoxicity. 5. Patients at risk for suicidal behaviour. 6. Immunocompromised patients. 7. Acute, or chronic, or recurrent medical conditions that might affect/jeopardize the study results. 8. Significant liver disease, defined as known active hepatitis or elevated liver enzymes \> 3 times the upper boundary of the normal ranges. 9. Value of creatinine outside the normal ranges and judged clinically relevant by Investigator. 10. Any contraindication listed in the Summaries of Product Characteristics (SmPC) of Carbolithium® and lithium sulphate prolonged-release tablets (Lithiofor®). 11. Positive history for drugs. 12. Alcohol abuse. 13. Positive urine drug screen for CNS-active drugs (cocaine, opioids, amphetamines and cannabinoids) at Visit 0 (screening). 14. Clinically significant abnormalities on physical examination, vital signs, ECG, laboratory tests prior to screening visit. 15. Inability to comply with the protocol requirements, instructions and study-related restrictions; e.g. uncooperative attitude, inability to return for study-visits, and improbability of completing the clinical study. 16. Vulnerable subjects (e.g. persons kept in detention). 17. Participation to an interventional clinical study within 3 months prior to the screening visit. 18. If Subject is the Investigator or his(her) deputies, first grade relative, research assistant, pharmacist, study coordinator, other staff or relative thereof directly involved in the conduct of the study.