Effect of lithium dosing schedule on urinary output.

Lithium has been reported to induce polyuria in up to 35% of patients receiving it. It has been suggested that polyuria may be reduced by using single rather than multiple daily dosing. However, this information is based on non-randomized studies, which used higher serum lithium levels than are currently used. In fact, the incidence of polyuria may be lower at currently used lithium levels, and the benefits of a single daily dose regimen on urine volume at these levels have not been assessed. We conducted a prospective randomized study to test the hypothesis that switching patients from multiple daily dose lithium to single daily dose lithium would significantly lower urine volume. Twenty-four patients previously stabilized on multiple daily dosing were randomly allocated to either single or multiple daily dose lithium. Twenty-four-hour urine volume, serum creatinine, creatinine clearance and serum lithium were measured at study entry and completion. Switching to single daily dose lithium did not significantly reduce the 24-h urine volume.