Inclusion Criteria:  Adults (age ≥ 18 years) Patient with bipolar disorder Patient treated with lithium for at least 5 years Patient with a urine concentration defect defined by a maximal urine osmolality < 600 mOsm/kg Woman of childbearing age agreeing to use an efficient contraceptive method for 12 months  Exclusion Criteria:  Renal failure defined as eGFR < 30 ml/min/1.73m² estimated by the CKD-EPI equation Hyperkalemia > 5 mmol/l Hypersensitivity or known allergy to amiloride Hypersensitivity to lactose Known adrenal insufficiency Concomitant use of other potassium-sparing treatment (e.g. spironolactone, angiotensin converting enzyme inhibitors (ACE), angiotensin II receptor (AT2R) antagonists, calcineurin inhibitors tacrolimus and ciclosporin) Acute ongoing infection (less than 3 days before inclusion) Severe heart failure (NYHA > II) Acute phase of mood disorder Previous use of amiloride (long term use ≥ 6 months or/and use in the 6 months prior to randomisation) Pregnant or breastfeeding women Participation in another clinical study involving investigational medicinal product or patient being in the exclusion period at the end of a previous study Patient refusal to participate Non-affiliation to a social security regimen or CMU Patient under State Medical Aid Subject deprived of freedom, subject under a legal protective measure