The study, published online in Bipolar Disorders, points to the enzyme, protein kinase C, known as PKC, as a target for new medications for mania. The anti-breast cancer drug Tamoxifen rapidly blocks PKC -- thought to be over-active during this phase of bipolar disorder, said researchers at the National Institutes of Health's National Institute of Mental Health in Bethesda, Md.

"We now have proof of principle," one of the study authors, Dr. Husseini Manji, said in a statement. "Our results show that targeting PKC directly, rather than through the trickle-down mechanisms of current medications, is a feasible strategy for developing faster-acting medications for mania."

Tamoxifen might not become a treatment because it also blocks estrogen -- which is why it is used to treat breast cancer -- and because use over long periods may cause endometrial cancer.

The three-week study involved 16 adults having a manic episode. By the end of the study, 63 percent of the patients taking Tamoxifen had reduced manic symptoms, compared with only 13 percent of those taking a placebo.