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The aim of this study was to compare treatment adherence and tolerability of different lithium formulations in 70 bipolar patients receiving lithium therapy for the first time. During the 1-year follow-up, information was collected regarding patient's clinical course, therapeutic adherence, side effects of the treatment and serum levels of lithium, creatinine and thyroid-stimulating hormone. At baseline, 30 patients (43%) were on prolonged-release lithium formulations and 40 (57%) on immediate-release formulations. At the final evaluation, 37 patients (53%) were considered lost to follow-up. Both prolonged- and immediate-release patients showed significant improvement in the Functioning Assessment Short Test and in the Clinical Global Impressions for Bipolar Disorder scores during the follow-up. At the first follow-up visit, the mean plasma lithium level of prolonged-release patients was higher than immediate-release patients (0.61 vs. 0.47, respectively; P = 0.063), as well as the therapeutic adherence (85 vs. 64%, respectively; P = 0.089). Fine tremor and gastrointestinal symptoms were more frequent in immediate-release patients than in prolonged-release patients at each follow-up visit, with the sole exception of gastrointestinal symptoms at the last evaluation. Prolonged-release lithium therapy could provide potential advantages over immediate-release formulations. Future naturalistic studies and clinical trials with a longer follow-up duration are needed.