The present study investigates the applicability of Fenton's oxidation for the treatment of a highly refractory, viscous, bulk drug industry effluent. The effluent is found to behave as a Newtonian fluid, thrice as viscous as water. Fenton's oxidation experiments on the undiluted effluent are conducted in accordance with central composite design (CCD), considering the operating factors of pH (3–11), FeSO4 dosage (3–15 g L−1), H2O2 dosage (7–19 mL L−1), and treatment time (30–150 min). The treatment yielded a maximum total organic carbon (TOC) reduction of 28% at neutral pH, FeSO4, and H2O2 dosages of 9 g L−1 and 13 mL L−1, respectively, for 150 min of operation. The maximal treatment efficacy observed at neutral pH is explained by the non-radical mechanism of Fenton's oxidation, and further correlated with viscosity and type of fluid (here, Newtonian), as judged from the generated viscosity profiles. Upon comparison with previous effluent samplings, differing treatment efficacies are observed for different fluid types, that is, Newtonian and non-Newtonian (shear thinning), with the latter obtaining twice the TOC reduction, under similar experimental conditions. Hence, the effect of viscosity and fluid type on the effectiveness of Fenton's oxidation treatment is experimentally validated and discussed. © 2021 Wiley-VCH GmbH