A glassy carbon electrode (GCE) was modified with graphite powder (G/GCE), and then treated with a solution of sodium peroxide for several minutes to prepare an oxidized G/GCE (OG/GCE). The OG/GCE has prominently improved responses toward dopamine (DA), rutin (RT) and acetamidophenol (APAP), of which the anodic peak current was increased by 2.4, 4.0 and 2.6 fold compared with that obtained using the G/GCE. The redox peaks of DA, RT and APAP on the OG/GCE could be separated sufficiently. The corresponding redox processes were confirmed as diffusion-controlled and parameters such as the charge transfer coefficients (α), saturating adsorption capacity (Γ*) and catalytic rate constant (kcat) were estimated. In individual detection, the linear ranges for DA, RT and APAP were 10 nM-10 μM, 1.00 nM-150 nM and 20 nM-30 μM, respectively, and the LODs for DA, RT and APAP were estimated as 6.23 nM, 0.36 nM and 13.1 nM with 3σ/S, respectively. The contents of RT and APAP in drugs were determined and agreed with the labeled contents. The recoveries of DA in serum and sweat were in the range of 91-107%, indicating that the determination results achieved by the OG/GCE are reliable. The practical use of the method was verified with a graphite-modified screen-printed carbon electrode (G/SPCE), which was also activated by Na2O2 to prepare OG/SPCE. The recovery of DA in sweat achieved using the OG/SPCE was 91.26%.