Tamoxifen, a selective estrogen receptor modulator (SERM) used in breast cancer therapy, requires metabolic activation by CYP3A4 to exert its biological effects. This study evaluated the effects of calcium channel blockers nimodipine, nitrendipine and felodipine on tamoxifen metabolism by studying their interactions with tamoxifen in vitro and in vivo. Rat liver microsomes (RLM) and human liver microsomes (HLM) were used in this study to evaluate the inhibitory potential of nimodipine, nitrendipine and felodipine on tamoxifen metabolism in vitro. A total of 28 cardiovascular drugs, including calcium channel blockers, were screened in an RLM incubation system in vitro. In RLM, nimodipine, nitrendipine and felodipine had half-maximum inhibitory concentration (IC