Oxycodone is a potent semi-synthetic opioid that is commonly used for the treatment of severe acute and chronic pain. However, oxycodone is also one of the opioids most implicated in drug abuse and addiction and contributes significantly to what is now being called the opioid epidemic in the US. Treatment with oxycodone can lead to cardiac electrical changes, such as long-QT syndrome, potentially inducing sudden cardiac arrest. Here, we investigate whether the cardiac side effects of oxycodone can be explained by modulation of the cardiac sodium channel Nav1.5. We stably expressed human Nav1.5 in HEK293 cells and used these for whole-cell patch-clamp electrophysiology. A variety of voltage-clamp protocols was used to test the effect of oxycodone on different channel gating modalities. To test the effect of oxycodone on cardiomyocyte beating, we used commercially available human stem cell-derived cardiomyocytes in combination with a CardioExcyte 96 device. Oxycodone concentration-dependently inhibits Nav1.5 with an IC50 of 483.2 µM. We show that this block is use-dependent, leading to a continuous run-down in Nav1.5 inward current over time, which is not the case in vehicle recordings. In addition, oxycodone slows both fast and slow time constants of recovery of Nav1.5 from fast inactivation. When we also tested slow inactivation of Nav1.5, we found that during vehicle treatment, approximately 25 % of channels entered the slow inactivated state with a slow time constant (mean ± SEM 16.8 ± 2.1 s; n = 8). During application of oxycodone, we observed an increase in the amount of slow inactivation as well as a speeding of the time constant. In this condition, 46 % of channels displayed slow inactivation with a significantly faster time constant (mean ± SEM 5.0 ± 0.5 s; n = 9). Finally, application of oxycodone even at 100 µM (far below the IC50) to spontaneously beating human stem cell-derived cardiomyocytes leads to a reduced beat rate and to arrhythmia. The basal beat rate of 58 ± 1 beats per minute was reduced by -21.5 ± 3.6 % (mean ± SEM in n = 5 wells) by oxycodone 100 µM, but not by vehicle (mean ± SEM +4.9 ± 1.7 % in n = 8 wells). The slope of the early downstroke of the extracellular field potential was strongly reduced by oxycodone, confirming a Nav1.5-mediated mechanism of action. Furthermore, the duration of the extracellular field potential was reduced by oxycodone, indicating an additional block of hERG potassium channels. Oxycodone leads to an accumulation of Nav1.5 in inactivated states with a slow time course. While the concentrations needed to elicit cardiac arrhythmia in vitro are comparably high, some patients under high-dose treatment with oxycodone as well as drug abusers might suffer from cardiac side effects induced by the slow effects of oxycodone on Nav1.5 and potential effects on other cardiac ion channels.