Publisher Summary This chapter discusses the side effects and treatment intensity of using Lithium. It examines the effect of Lithium dosage on organs and systems like cardiovascular system, neuromascular system, effects of lithium on memory, thyroid, parathyroid, metabolic functions, hematological functions, gastrointestinal system, urinary system, and skin and hair. It also explains the lithium intoxication, interactions, and miscellaneous effects of lithium administration. This chapter discusses some publications on the effects of long-term lithium treatment on thyroid function, renal function, erythrocyte folate concentration, fasting blood sugar, and seasonal variations of blood pressure. Prediction of drop-out problems of non-compliance, and the effect of lithium treatment on sexual functions have also been analyzed. It explains the reactions of lithium dosage on test individuals and notes that the physician should spend time and effort on adjusting lithium doses and serum concentrations to those levels which for each patient give a maximum prophylactic effect with a minimum of side effects.