Due to the wide interindividual variability in pharmacokinetics and therapeutic response of cancer patients towards anticancer drugs, pharmacogenomic-assisted dose optimization and/or application of therapeutic drug monitoring service could be of benefit. For some drugs like irinotecan used in solid tumors, dose optimization may be achieved if the oncologist depends on pharmacogenomic approach. On the other hand, dose adjustment of some other anticancer drugs like tamoxifen used in breast cancer can be affected more by application of therapeutic drug monitoring of its active metabolite; endoxifen.