The utilization of tamoxifen is considerably high in Indonesia, with about 170,000 tamoxifen prescriptions filed in 2015. It is metabolized by the enzyme CYP2D6, resulting in its active metabolite, endoxifen, which has been proven to be effective in the prevention and treatment of breast cancer.

Studies showed the CYP2D6 gene has more than 100 variants; some of which are linked with reduced drug activity, while others do not have any pathological implications. The metabolizer profile of these variants is generally grouped into Ultra-rapid, Normal, Intermediate, and Poor Metabolizers (UM, NM, IM, and PM, respectively). In our previous study (NCT04312347), the investigators recruited 150 breast cancer patients who were taking adjusted dose of tamoxifen daily based on their CYP2D6 phenotype. Although the investigators have measured the endoxifen level of the patients with adjusted treatment, the clinical outcomes of the study are not yet conclusive.

Inclusion Criteria:

1. female
2. diagnosed with ER+ breast cancer
3. have been genotyped and classified as PM and IM in the previous study
4. are recommended by doctor to take tamoxifen 40 mg according to their metabolizer profile
5. have finished the definitive therapy course (surgery, chemotherapy, or radiotherapy).

Exclusion Criteria:

1. have other primary cancer aside from breast cancer.
2. those with residual tumor cells/have experienced second primary breast tumor.
3. patients who are recommended by doctor to switch to aromatase inhibitors (AI)