We formerly reported the most suitable volumes of medicinal solution for continuous 5FU infusion over 46 hours in the FOLFOX6 and FOLFIRI regimens, respectively, by analyzing the relation of the total volume of the medicinal solution in a portable disposable infusion pump (SUREFUSER A) and the duration of infusion using a regression analysis. A total infusion time of about 48 hours was obtained. As the ambient temperature increased, however, we noticed that the continuous 5FU infusion finished earlier than anticipated. Using an additional analysis, we found that not only the medicinal solution's original coefficient of viscosity, but also the ambient temperature influenced the duration of infusion. Here, we report that the speed of continuous 5FU infusion increases as the coefficient of viscosity decreases in response to increases in ambient temperature. Thus, the composition of medicinal solutions and the ambient temperature must be considered to ensure a correct duration of continuous infusion.