Detection of traces from pharmaceutical drugs such as antibiotics in drinking water and foodstuff is essential for guaranteeing human health in some environments, and this has to be done with low cost technologies to be widely deployed in public services and industry. In this paper, we describe an ultralow cost (<US$ 0.005 per unit of sensing layer) electrochemical sensor to detect the antibiotic tetracycline, which is made of a homogeneous thin film of potato starch (PS) and carbon black (CB) deposited on glassy carbon electrodes (GCE). Detection of tetracycline was also performed in real samples consisting of tap water, river water, milk and in solutions prepared with commercial tablets of this medicine in the range between 5.0 and 120 μmol L⁻¹, with a detection and quantification limit of 1.15 and 4.47 μmol L⁻¹, respectively. The high sensitivity was attributed to the enhanced conductivity and larger surface area induced by incorporating the carbon black into potato starch. The CB‐PS/GCE electrodes were reproducible and stable, thus serving as a generic platform for detection of other antibiotics and hormones whose redox potentials are similar to those of tetracycline.