Breast cancer risks following antipsychotic use in women with bipolar disorder versus schizophrenia: A territory-wide nested case-control study spanning two decades.

Accrued epidemiologic data largely support an association of antipsychotic use with breast cancer in women with schizophrenia. No studies have specifically investigated such risks in women with bipolar disorder. This study aims to examine the association between antipsychotics and breast cancer in women with bipolar disorder and compare it against schizophrenia. We conducted a nested case-control study using a territory-wide public healthcare database in Hong Kong examining women aged ≥18 years with bipolar disorder or schizophrenia. Using incidence density sampling, women with a breast cancer diagnosis were matched by up to 10 control participants. In total, 672 case participants (109 with bipolar disorder) and 6,450 control participants (931 with bipolar disorder) were included. Results show a significant association of first-generation antipsychotics with breast cancer in both women with schizophrenia [adjusted odds ratio (aOR) 1.49, 95% confidence interval (CI) 1.17-1.90] or bipolar disorder (aOR 1.80, 95% CI 1.11-2.93). Second-generation antipsychotics was associated with breast cancer only in women with bipolar disorder (aOR 2.49, 95% CI 1.29-4.79), with no significant association found in women with schizophrenia (aOR 1.10, 95% CI 0.88-1.36). In conclusion, further research on breast cancer risks is warranted for women with bipolar disorder on antipsychotics.