Effect of lithium on plasma chlorpromazine levels

The interaction of lithium and chlorpromazine (CPZ) was studied in healthy volunteers in a randomized crossover study. Each subject ingested two doses of CPZ (100 mg) as the liquid concentrate: (1) without concurrent lithium therapy and (2) after a 7-day treatment with lithium carbonate (900 mg/day). When CPZ was administered with lithium, peak plasma CPZ levels were 40.3% (mean) lower than those without lithium (p = 0.006), and the area under the CPZ plasma concentration time curve was 26.6% smaller (p = 0.08). The time to reach peak plasma CPZ levels was similar in both groups. All subjects slept for 4 to 6 hr after oral CPZ and had a maximum fall in both systolic (8 to 32 mg Hg) and diastolic (5 to 23 mg Hg) blood pressure at the time of peak plasma CPZ concentration. This lithium-CPZ interaction may explain the low plasma CPZ levels reported previously in psychiatric patients taking both lithium and CPZ.