Purpose: Despite evidence supporting at least five years of endocrine therapy for early breast cancer, many women discontinue therapy early. We investigated the impact of initial therapy type and specific comorbidities on discontinuation of endocrine therapy in Australian clinical practice.

Methods: We identified women in the New South Wales 45 and Up Study with a diagnosis of invasive early breast cancer and an incident dispensing of anastrozole, letrozole or tamoxifen from 2003-2008 (N=1531). Linked pharmacy, hospital and health service data were used to determine endocrine therapy duration (up to five years), treatment for pre-existing and post-initiation comorbidities (anxiety, depression, hot flashes, musculoskeletal pain, osteoporosis, vaginal atrophy), demographic and other clinical characteristics. Time to discontinuation of initial, and any, endocrine therapy was calculated. Cox regression determined the association of different characteristics on early discontinuation.

Results: Initial endocrine therapy continued for a median of 2.2 years and any endocrine therapy for 4.8 years. Cumulative probability of discontinuing any therapy was 17% after one year and 58% by five years. Initial tamoxifen, pre-existing musculoskeletal pain and newly-treated anxiety predicted shorter initial therapy but not discontinuation of any therapy. Early discontinuation of any therapy was associated with newly-treated hot flashes (HR=2.1, 95%CI=1.3-3.3), not undergoing chemotherapy (HR=1.4, 95%CI=1.1-1.8) and not undergoing mastectomy (HR=1.5, 95%CI=1.2-1.8).

Conclusions: Less than half of women completed five years of endocrine therapy. Women at greatest risk of stopping any therapy early were those with newly-treated hot flashes, no initial chemotherapy, or no initial mastectomy. This suboptimal use means that the reductions in recurrence demonstrated in clinical trials may not be realised in Australian practice.