DayQuil and NyQuil are commonly used to suppress cold and flu symptoms. The purpose of this scientific investigation is to determine the effect of these commonly used medicines on the heart rate of a living organism. More specifically, Daphnia magna will be exposed to various concentrations of DayQuil and NyQuil and the heart rate will be measured. Four concentrations of each DayQuil and Nyquil were prepared in the strengths of 15 mL (half the recommended dose), 30 mL (recommended dose), and 60 mL (double the recommended dose) and diluted with 500 mL of spring water, and compared to the control, spring water. One at a time, an initial heart rate was calculated for a Daphnia magna. The Daphnia magna was then exposed to one of the four concentrations for two minutes; the heart rate was calculated, and the change in heart rate was derived and recorded. After eight trials for each concentration and the control were conducted, it was determined that the exposure to both DayQuil and NyQuil had an inverse relationship between the concentrations and the heart rate of the Daphnia magna, as the concentration increased the heart rate decreased.