Unexpected interaction between quetiapine and valproate in patients with bipolar disorder.

Quetiapine, a second-generation antipsychotic originally used in the treatment of schizophrenia, was also found to be effective as monotherapy or as an adjunctive therapy for acute mania. Delirium in patients treated with quetiapine seems to be a rare phenomenon; however, we report two patients with bipolar disorder who developed delirium when prescribed quetiapine as an adjunct to valproate for acute mania. Both had previously developed mild renal insufficiency after an episode of lithium intoxication. The delirium resolved after quetiapine was discontinued. Unexpected interactions may occur when medications are combined without being subject to controlled clinical trials.