Tamoxifen is a Selective Estrogen-Receptor Modulator (SERM) commonly prescribed for standard of care in estrogen receptor positive (ER+) breast cancer as an adjuvant therapy. Tamoxifen is metabolized by CYP2D6 into its active metabolite, endoxifen, which has been known to play an important role in reducing risk of ER+ breast cancer recurrence. CYP2D6 is a highly polymorphic gene with more than 100 alleles. The phenotype of this gene is categorized into ultrarapid metabolizer (UM), normal metabolizer (NM), intermediate metabolizer (IM), and poor metabolizer (PM). Certain CYP2D6 polymorphisms may cause reduced activity of this enzyme. Studies have found that reduced CYP2D6 activity in IM and PM patients causes low efficacy of standard tamoxifen therapy. This study aims to observe the distribution of CYP2D6 alleles and its correlation with endoxifen levels in Indonesian ER+ breast cancer patients. 151 patients who have received tamoxifen therapy for at least eight weeks were recruited prospectively. DNA and blood samples were collected with buccal swab and finger-prick methods, respectively. Genotyping was performed using the qPCR method while metabolite level measurement was performed using high performance liquid chromatography tandem mass spectrometry. We found that 40.67% of ER+ breast cancer patients recruited were IM. CYP2D6*10 was the most abundant allele (0.288) in this population, and *10/*36 was the most frequently observed diplotype (0.236). Endoxifen levels between the NM-PM, NM-IM, and IM-PM were statistically significant (p-value = 6.26 x 10-5, 9.12 x 10-5, and 4.714 x 10-3, respectively), and dose increase of tamoxifen to 40 mg daily successfully increased endoxifen levels in IMs to a similar level with NMs at baseline. Given these findings, implementing pharmacogenomic testing of CYP2D6 on ER+ breast cancer women who are about to undergo tamoxifen therapy may be beneficial to increase the likelihood of achieving expected endoxifen levels, thus better treatment efficacy.