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BACKGROUND The presence of micro‐pollutants in conventional wastewater treatment plants raises environmental concerns due to their insufficient removal. Non‐biological methods are needed to treat such compounds and this work demonstrates the use of electrochemical oxidation for the degradation of the drug dexamethasone (DEX). RESULTS A cell consisting of a boron‐doped diamond anode and a stainless steel or carbon cloth cathode was employed, with Na2SO4 being the electrolyte. DEX degradation rate, at the low level of mg L−1 (0.25–2 mg L−1), increases with increasing current (0.02–0.2 A m−2), as well as in the presence of sodium persulfate (50–250 mg L−1); the latter is electrochemically activated to produce additional oxidants and enhances degradation in a synergistic way. A rate constant of 0.194 min−1 was recorded for 0.5 mg L−1 DEX degradation at 0.2 A m−2 with 150 mg L−1 persulfate. The water matrix had little effect on performance compared to experiments in pure water; the only exception was the presence of chloride (50–250 mg L−1), which substantially improved rates due to the formation of active chlorine species (up to about five times). Carbon cloth accelerated DEX removal by up to three times compared to stainless steel and this was partly due to its adsorptive capacity; similarly, reactions involving DEX transformation products (TPs) also occurred faster. Seven TPs were identified and profiles were followed; reaction pathways involve hydroxylation, oxidation, decarboxylation and ring‐opening reactions, followed by fluorine release in the liquid phase. CONCLUSION A hybrid technology based on electrochemical oxidation and persulfate activation is proposed for the treatment of residual pharmaceuticals in environmental matrices. © 2021 Society of Chemical Industry (SCI).