OBJECTIVES: Adherence to oral medications among patients with breast cancer (BC) has been reported to be suboptimal. We conducted a systematic literature review to assess the economic burden due to treatment non-adherence among patients with BC. METHODS: Embase^® and MEDLINE^® were searched for the relevant studies published post 2005. Each citation was reviewed by two independent reviewers; any disagreement was resolved by a third reviewer. RESULTS: Of the 595 citations obtained from the database search, three studies met the inclusion criteria. Adherence to hormone therapy (two studies) was measured by proportion of days covered and to lapatinib (one study) by medication possession ratio, time to treatment interruption/discontinuation and time to end of continuous treatment. The proportion of non-adherent patients ranged from 22% to 46% across the included studies. In an economic evaluation, low adherence to tamoxifen was found to be associated with an increased discounted medical cost of £5,970 (95%CI: £4,644–£7,372) over an average patient’s lifetime. This study used data for women with incident BC between 1993 and 2000 in the Tayside region of Scotland. Key contributors to increased cost were in-patient stay and other dispensing costs. Similarly, among patients with non-metastatic BC, medical costs in the adherent group with tamoxifen and aromatase inhibitors appeared to be lower than in the non-adherent group during the 4-year study period (difference between non-adherent and adherent: $1,476 [year 1], $8,008 [year 2], $-290 [year 3], and $1,857 [year 4]). Adjusted medical costs were reported to be 31% less for adherent cohort. Among women with metastatic BC, a statistically non-significant association was reported between non-adherence to lapatinib and total healthcare costs (p=0.870). CONCLUSIONS: Limited published evidence suggested that treatment non-adherence tends to increase medical and overall healthcare costs in BC. Improvement in adherence can help in reducing the economic burden in long-term and improve cost-effectiveness.