Lithium is used in the long-term treatment of bipolar disorder, exhibiting a beneficial effect on the neuronal cells. The concentration of lithium in the blood serum can vary and can easily approach a level that is related to cardiotoxic adverse effects. This is due to its narrow therapeutic index. In this study, we investigated the effect of higher than therapeutic dose of lithium. Rat cardiomyoblast cells were treated with 2 mM LiCl for 48 h, after which the mitochondrial parameters of the cells were analyzed. Lithium exposure reduced maximal respiratory capacity by diminishing reserve respiratory capacity (RRC), linked to a decrease in complex I (NADH dehydrogenase) activity and elevated superoxide radical levels. In addition, lithium treatment altered the composition of cellular membranes, including mitochondrial cardiolipin, a lipid essential for mitochondrial function. These findings suggest that impaired complex I activity, oxidative stress, and cardiolipin depletion collectively impair the ability of cells to meet high energy demands.