The current treatment of breast carcinomas recognizes the importance of combination therapy in order to increase efficacy and decrease side effects of conventional chemotherapy. Inositol hexaphosphate (IP6), a naturally occurring polyphosphorylated carbohydrate, has shown a significant anti-cancer effect in various in vivo and in vitro models, including breast cancer. In this study, we investigated the in vitro growth inhibitory activity of IP6 in combination with adriamycin or tamoxifen, against three human breast cancer cell lines: estrogen receptor (ER) alpha-positive MCF-7, ER alpha-negative MDA-MB 231 and adriamycin-resistant MCF-7 (MCF-7/Adr) using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. Much lower concentrations of IP6 were required after 96 h of treatment to inhibit the growth of MCF-7/Adr cells than MCF-7 cells; the IC50 for MCF-7/Adr cells was 1.26 mM compared to 4.18 mM for MCF-7 cells. The ER-negative MDA-MB 231 cells were also highly sensitive to IP6 with IC50 being 1.32 mM. To determine the effects of IP6 in combination with either adriamycin or tamoxifen, the median effect principle and Webb's fraction method were used to determine the combination index (CI) and the statistical differences. Growth suppression was markedly increased when IP6 was administered prior to the addition of adriamycin, especially against MCF-7 cells (CI = 0.175 and p < 0.0001). Synergism was also observed when IP6 was administered after tamoxifen in all three cell lines studied (CI = 0.343, 0.701 and 0.819; p < 0.0001, p = 0.0003 and 0.0241 for MCF-7/Adr, MCF-7 and MDA-MB 231, respectively). The growth of primary culture of breast cancer cells from patients was inhibited by IP6 with LC50 values ranging from 0.91 to 5.75 mM (n = 10). Our data not only confirm that IP6 alone inhibits the growth of breast cancer cells; but it also acts synergistically with adriamycin or tamoxifen, being particularly effective against ER alpha-negative cells and adriamycin-resistant cell lines.