We investigated the effect of anodic treatment of titanium/tetrahedral amorphous carbon electrodes on the electrochemical detection of morphine and paracetamol. The anodic treatment caused both oxidation of the carbon and, more importantly, exposure and oxidation of the underlying Ti layer. This treatment anodically shifted the oxidation potential of paracetamol while that of morphine remained unaffected. The resulting electrode also showed better selectivity than a ta-C electrode without Ti. After anodic treatment at 2.5 V, selective detection of morphine with a physiologically meaningful detection limit of 9.8 nM and a linear range of 0.1–10 μM was obtained in the presence of 100 μM paracetamol.