Photochemical Transformations in the Management of Psychoactive Drug Residues: Fate, Treatment, Monitoring and Safety

Psychoactive drugs used in clinical practice or consumed illicitly enter interconnected systems of pharmaceutical handling, human exposure, sewer transport, wastewater treatment and receiving waters. Light-driven chemistry is relevant across this continuum, but its significance changes with the endpoint: preservation of medicinal products, environmental transformation, engineered contaminant removal, wastewater-based monitoring or prevention of photoactivated harm. This critical narrative review evaluates photochemical processes affecting antidepressants, benzodiazepines, antipsychotics, neuroactive anticonvulsants and selected illicit-drug markers, with emphasis on literature published from 2000 to 2 September 2026 and historically important earlier studies. Evidence was examined for reaction mechanisms, environmental realism, transformation products, ecotoxicological outcomes and translational maturity. Direct photolysis varies markedly among compounds and irradiation conditions; dissolved organic matter and inorganic solutes may accelerate or suppress transformation through competing optical and radical pathways. Ultraviolet/oxidant systems, photo-Fenton chemistry and heterogeneous photocatalysis frequently accelerate parent-compound disappearance, but this endpoint does not establish mineralisation, ecological safety or economical operation. Carbamazepine illustrates the central contradiction: removal of the parent compound may coincide with formation of acridine-related products and transient increases in mixture toxicity. Experimental work on sertraline, citalopram, benzodiazepines and phenothiazines confirms that photoproduct identity and bioactivity demand compound-specific scrutiny. Evidence involving cocaine shows why environmental transformations also matter to forensic and wastewater-epidemiological interpretation. The literature remains dominated by controlled laboratory studies, a limited range of benchmark drugs and inconsistent reporting of absorbed light, transformation-product uncertainty and life-cycle burdens. A defensible management framework therefore treats light chemistry as a conditional tool for source control, monitoring and water treatment, evaluated against the combined criteria of chemical conversion, residual hazard, operational feasibility and reliable surveillance rather than degradation percentage alone.