Diabetes Drug Sitagliptin Reveals Hidden Gaps in Water-Reuse Risk Assessment

A widely prescribed diabetes drug long dismissed as environmentally harmless is now at the center of a debate about how scientists judge pharmaceutical pollution in farmland irrigated with recycled wastewater. Sitagliptin, a blockbuster medication taken by millions of people with type 2 diabetes, has consistently passed conventional aquatic risk assessments because the concentrations measured in rivers and lakes sit far below levels that harm aquatic life. But a new critical review published in Environmental Science and Pollution Research argues that this reassuring conclusion is only part of the story, and that the standard assessment framework may be systematically blind to the ways drugs behave once treated wastewater reaches agricultural soils.

The review, authored by Mohammed F. Hamdi of the University of Al Maarif in Iraq, does not claim that sitagliptin is a proven hazard. Instead, it uses the compound as a sentinel, a model case for stress-testing the dominant tool of pharmaceutical environmental regulation: the comparison of a predicted environmental concentration against a predicted no-effect concentration, known as the PEC/PNEC ratio. When that ratio stays below one, regulators typically conclude the risk is acceptable. The problem, Hamdi argues, is that this calculation is built around the parent compound in surface water, while real-world exposure in water-reuse agroecosystems unfolds in soil, in the presence of organic matter, biosolids, mixtures of other drugs, and a shifting cast of transformation products that the standard framework never accounts for.

The scale of the underlying exposure problem is well documented. Pharmaceuticals are excreted partially unchanged, pass through conventional wastewater treatment with variable efficiency, and enter rivers, groundwater, and irrigation systems worldwide. Global surveys have detected hundreds of active pharmaceutical ingredients in the waterways of every inhabited continent, and reclaimed wastewater is increasingly promoted as a strategic resource in water-stressed regions. The European Union has even adopted minimum requirements for water reuse in agriculture, formalizing a practice that delivers not only water but also trace organic chemicals, including antidiabetic drugs, directly onto cropland.

What makes sitagliptin especially interesting is what happens to it in soil. Direct experimental evidence shows that its retention in soil is strongly matrix-dependent, meaning it varies with soil organic matter content and pH. More strikingly, co-exposure to biosolids, sewage effluent, and metformin, another ubiquitous antidiabetic drug, can reduce sitagliptin sorption under tested conditions. In practical terms, the very amendments farmers apply to enrich their soils, and the very co-contaminants that travel with treated wastewater, may loosen the drug’s grip on soil particles and plausibly increase its mobility. A compound that looks stationary and benign in a simple laboratory sorption test may behave quite differently in a biosolids-amended field receiving effluent-dominated irrigation water.

This mobility question matters because dissolved organic matter, or DOM, acts as a chemical shuttle in soil systems. Organic molecules derived from manure, sludge, and wastewater can bind pharmaceuticals and carry them through soil profiles, a mechanism demonstrated for other drugs such as sulfadiazine, caffeine, and atenolol. Soil column experiments have confirmed that contaminants of emerging concern can move through soil under realistic flow conditions, and studies of riverbed sediments show that pharmaceuticals interact with benthic biofilms and suspended particles in ways that simple water-column models ignore. For a polar, relatively mobile compound like sitagliptin, these facilitation pathways could extend its residence and reach well beyond what a PEC calculated from river concentrations would suggest.

Field-scale studies of reclaimed-water irrigation do support the idea that soils accumulate pharmaceutical residues over time. Research on processing tomato and wheat successions irrigated with treated wastewater has documented accumulation of emerging contaminants in the soil-plant system, and large surveys, notably in Israel where wastewater reuse is extensive, have detected pharmaceuticals in edible crops. Yet the review is careful about what this evidence does and does not show: field studies support soil exposure and accumulation for sitagliptin, but they do not demonstrate edible-crop contamination or food-chain risk for this specific drug. That distinction is crucial. The absence of demonstrated dietary exposure is not proof of safety, but neither is soil accumulation proof of harm, and the review resists both overstatements.

Perhaps the most consequential finding concerns transformation products, the chemical offspring of pharmaceuticals that form during treatment and environmental degradation. Sitagliptin is not destroyed by conventional biological treatment so much as biologically transformed, and advanced processes such as ozonation generate a suite of oxidative transformation products whose identities, kinetics, and pathways have been mapped in laboratory and full-scale studies. Ultraviolet irradiation likewise produces characteristic degradation products. Crucially, attenuation of the parent compound in a treatment plant or a river does not equal risk closure. The transformation products may persist, may be mobile, and may carry toxicity of their own, and in most regulatory assessments they are simply not evaluated. Recent work on hyporheic zones, the ecologically active sediments beneath and beside rivers, adds nuance: sequential anoxic and oxic conditions there can biotransform sitagliptin and reduce its toxicity, suggesting that some natural systems genuinely degrade the drug rather than merely relocating or reshaping it.

The mixture dimension compounds the uncertainty. Wastewater-impacted waters never contain a single drug in isolation; they carry cocktails of antidiabetics, antibiotics, analgesics, and their metabolites, alongside dissolved organic matter that modifies both transport and bioavailability. Environmental risk assessment of pharmaceutical mixtures remains one of the field’s most persistent gaps, with recognized demands for combined-toxicity approaches that current regulatory practice rarely meets. Metformin, the world’s most prescribed diabetes drug and a frequent companion of sitagliptin in effluent, has itself become a poster child for this problem, since its transformation product guanylurea is persistent and widespread yet historically absent from risk calculations. The sitagliptin review explicitly frames mixture uncertainty as a reason why a low PEC/PNEC ratio for one parent compound cannot automatically close the file.

So where does this leave the drug itself? The review’s conclusion is deliberately measured: sitagliptin is not reclassified as a proven high-risk contaminant. Its aquatic risk assessment for the parent compound remains valid within its own terms. What the analysis challenges is the inference that those terms are sufficient. When land application of biosolids, irrigation with reclaimed water, DOM-facilitated transport, co-occurring pharmaceuticals, and incompletely characterized transformation products all reshape exposure, a clean aquatic assessment of the parent molecule says little about what happens in an irrigated field or a hyporheic sediment. The author’s proposal is not alarm but expansion: environmental evaluation should not end at the water’s edge, and low parent-compound risk should not function as a certificate of overall safety.

The broader lesson extends well beyond one diabetes medication. As water reuse expands globally and pharmaceutical consumption rises with aging populations, the chemicals that slip through treatment plants will increasingly land on the soils that grow our food. Sentinel compounds like sitagliptin expose the structural blind spots of a regulatory system designed for a simpler exposure world, and they point toward the next generation of assessment tools: soil-aware fate models, mixture toxicity frameworks, transformation product screening, and monitoring that follows drugs from effluent through root zone to crop. Whether regulators adopt that broader lens may determine whether the quiet accumulation of pharmaceuticals in reused-water agroecosystems remains a scientific curiosity or becomes one of the defining contamination stories of the coming decades.

Subject of Research: Environmental fate and risk assessment of the antidiabetic drug sitagliptin in water-reuse agroecosystems

Article Title: Sitagliptin beyond low aquatic risk: mobility, transformation products, and mixture uncertainty in water-reuse agroecosystems

Article References: Hamdi, M. F. (2026). Sitagliptin beyond low aquatic risk: mobility, transformation products, and mixture uncertainty in water-reuse agroecosystems. Environmental Science and Pollution Research. https://doi.org/10.1007/s11356-026-38297-4

Image Credits: AI Generated

DOI: 10.1007/s11356-026-38297-4

Keywords: sitagliptin, pharmaceutical pollution, reclaimed wastewater, soil sorption, transformation products, biosolids, dissolved organic matter, metformin, crop uptake, hyporheic zone, risk assessment, water reuse