Side-effects of lithium at lower therapeutic levels: the significance of thirst.

The prevalence of thirst, subjective polyuria and related side-effects was investigated in 87 patients attending a lithium clinic and in a group of 52 controls. Thirst was surprisingly common, occurring in 67% of patients, in spite of the fact that they had been maintained on relatively low levels of lithium, and was due principally to the lithium rather than to other psychotropic drugs. Urine flow and impaired renal water absorption correlated with the serum lithium level and the length of treatment in the patients, despite the fact that few were clinically polyuric. The pattern of the results confirms previous suggestions that lithium may stimulate the thirst mechanism directly as well as via an increased renal resistance to vasopressin. The possible implications in terms of clinical response are discussed.