Rapid and effective differentiation and quantification of a small molecule drugs, such as fentanyl, in bodily fluids is one of the major challenges for diagnosis and personal medication. Current toxicology methods used to measure drug concentration and metabolites require laboratory-based testing, which is not an efficient or cost-effective way to treat patients in a timely manner. Here, we show a universal assay for monitoring fentanyl levels by combining the intermolecular interaction enabled single-molecule recognition (iMSiR) with differential impedance analysis of conjugated polymers. The differential interaction with the designed anchor interface is transduced through manipulating the electronic status of the flexible conducting polymer. This assay showed excellent fentanyl selectivity against common interferences presented in the variable body fluids through either testing strips or skin patches. Directly using the patient's blood, the sensor provided 1-5% of the average deviation when compared to the gold standard of LC-MS in the medically relevant fentanyl range of 20 nM-90 nM. The superior sensing properties in conjunction with mechanical flexibility and compatibility enabled the universal point of care detection and provided a promising avenue for that can be deployed in the areas beyond the scope of biomarker detection.