Rapid and accurate detection of tetrahydrocannabinol (THC) and its main secondary metabolite carboxy-tetrahydrocannabinol (THC-COOH) is important to ensure safe roadways and workplaces, particularly in regions of the world where cannabis use is legal. In this work, we seek to demonstrate the usefulness of electrochemical SERS (EC-SERS) for the rapid detection of both THC and THC-COOH, complemented by thorough ab initio calculations for both molecules. These results indicate that application of a voltage is essential for efficient SERS detection of cannabinoids at low concentrations in bodily fluids, allowing for the eventual development of sensitive and quantitative screening tools. To the best of our knowledge, this work represents the first EC-SERS study of both THC and THC-COOH.