Many antipsychotics elevate prolactin; however, no studies have investigated antipsychotic use in patients with breast cancer. This study investigated if antipsychotic use is associated with an increased risk of cancer-specific mortality among breast cancer patients.
A cohort of 23,695 women newly diagnosed with a primary breast cancer between 1 January 1998 and 31 December 2012 was identified from the UK Clinical Practice Research Datalink linked to English cancer-registries and followed until 30 September 2015.
In total, 848 patients were prescribed an antipsychotic, of which 162 died due to their breast cancer. Compared with non-use, antipsychotic use was associated with an increased risk of breast cancer-specific mortality (HR, 2.25; 95% CI, 1.90-2.67). However, this did not follow a dose-response relation.
Analyses restricting the cohort to patients with a history of severe mental illness and analyses comparing prolactin-elevating and prolactin-sparing antipsychotics all revealed null associations.
Taken together, these results appear to suggest that the associations observed are a result of confounding by indication that patients with severe mental illness are at increased risk of breast cancer-specific mortality and that these patients are more likely to receive antipsychotics.
The study authors say the study provides an exemplar of confounding by indication, highlighting the importance of consideration of this important bias in studies of drug effects in cancer patients.