Environmental Concentrations of the Type 2 Diabetes Medication Metformin and its Transformation Product Guanylurea in Surface Water and Sediment in Ontario and Quebec, Canada.

Metformin, used to treat Type 2 Diabetes, is the active ingredient of one of the most prescribed drugs in the world, with over 120 million yearly prescriptions globally. In wastewater treatment plants (WWTPs), metformin can undergo microbial transformation to form the transformation product guanylurea, which could have toxicological relevance in the environment. Surface water samples from 2018-2020 and sediment samples from 2020 were collected from 6 mixed-use watersheds in Quebec and Ontario, Canada and analyzed to determine the metformin and guanylurea concentrations at each site. Metformin and guanylurea were present above their limits of quantification in 51.0 and 50.7% of all water samples, and in 64% and 21% of all sediment samples, respectively. In surface water, guanylurea was often present at higher concentrations than metformin, while the inverse was true in sediment, with metformin frequently detected at higher concentrations than guanylurea. Additionally, at all sites influenced solely by agriculture, concentrations of metformin and guanylurea were lower than 1 µg/L in surface water, suggesting that agriculture is not a significant source of these compounds in the investigated watersheds. These data suggest that WWTPs and potentially septic system leaks are the most likely sources of the compounds in the environment. Guanylurea was detected at many of these sites above environmental concentrations of concern, where critical processes in fish may be affected. Due to the scarcity of available ecotoxicological data and the prominence of guanylurea across all sample sites, there is a need to perform more toxicological investigations of this transformation product and revisit regulations. This report will help provide toxicologists with environmentally relevant concentration ranges in Canada.