The clinical benefit of implementing Bayesian approach for lithium drug monitoring was evaluated. Intervention group (N = 42) and historical control group (N = 55) patients were each divided into two groups: Dosage with immediate-release lithium carbonate or a sustained-release formulation, lithium citrate. Bayesian approach was performed in the intervention groups, and estimation of lithium steady-state trough concentration was obtained from non-steady-state blood sample, collected about 12 hr after the first lithium study dose. The estimate was compared with the actually measured steady-state concentration. In the control group, lithium monitoring was traditionally performed as steady-state blood sampling. Predicted and measured lithium concentrations were comparable. The desired lithium dose was reached significantly faster in the intervention group compared to control; 2.47 ± 2.22 days versus 9.96 ± 11.24 days (mean ± S.D.) (p = 0.0003). Bayesian approach was an advantage for the clinicians as a fast and safe aid to obtain the optimal lithium treatment dose.