Both human and veterinary medicine have relied on the antibiotic sulfamethoxazole to treat infections. However, high consumption of this compound culminates in environmental and public health problems. In this context, here we describe a voltammetric method to determine sulfametoxazole levels, using a glassy carbon disc electrode, chemically modified with nickel hexacyanoferrate film. We prepared the by cyclic voltammetry. The results provided a linear correlation coefficient (r) of 0.998 with a standard deviation (SD) of 0.053 μA, a limit of detection (based on the 3 SD/m ratio) equal to 6.8 nmol L-1, and a limit of quantification (based on the 10 SD/m ratio) of 22.7 nmol L-1.