There are contradictory results in the literature relating to the effect of tamoxifen on membrane fluidity. The present work investigates the effect of tamoxifen on membrane dynamics to find out whether the concentration of tamoxifen can be one of the factors in this discrepancy. Turbidity (absorbance at 440 nm) and Fourier transform infrared spectroscopic studies reveal that tamoxifen causes opposite effects on membrane fluidity at low (1 mol.%) and high (30 mol.%) tamoxifen concentrations. Low tamoxifen concentrations increase the absorbance in the gel and liquid crystalline phase, whereas high tamoxifen concentrations decrease the absorbance in gel and liquid crystalline phase, whereas tamoxifen concentrations decrease the absorbance. Observations on both phases show that the bandwidth of the CH2 stretching bands decreases with 1 mol.% tamoxifen and increases with 30 mol.% tamoxifen present, indicating a decrease in membrane fluidity at low tamoxifen concentrations and an increase in fluidity at high tamoxifen concentrations. It is seen that the apparent discrepancy in the literature on the effect of tamoxifen on membrane fluidity mainly arises from the tamoxifen concentration used and the confusion on the concept of lipid fluidity and lipid order.