The toilet is the primary venue where the Metformin enters the aquatic environment. Metformin is a pharmaceutical used by the growing population suffers from diabetes in Mexico City. Stable in the human metabolic system, the Metformin usually released unchanged through human waste and entered into the soil through sewer system. Although Metformin is biodegradable in soil, its transforming product, Guanylurea, is more stable and more challenging to be broken down.

Lesser, Luis E, Abrahan Mora, Cristina Moreau, Jürgen Mahlknecht, Arturo Hernández-Antonio, Aldo I Ramírez, and Héctor Barrios-Piña. "Survey of 218 Organic Contaminants in Groundwater Derived from the World's Largest Untreated Wastewater Irrigation System: Mezquital Valley, Mexico." Chemosphere 198 (2018): 510-21.

Poursat, Baptiste A.J, Rob J.M Van Spanning, Martin Braster, Rick Helmus, Pim De Voogt, and John R Parsons. "Biodegradation of Metformin and Its Transformation Product, Guanylurea, by Natural and Exposed Microbial Communities." Ecotoxicology and Environmental Safety 182 (2019): 109414.
Straub, Jürg Oliver, Daniel J Caldwell, Todd Davidson, Vincent D'Aco, Kelly Kappler, Paul F Robinson, Brigitte Simon-Hettich, and Joan Tell. "Environmental Risk Assessment of Metformin and Its Transformation Product Guanylurea. I. Environmental Fate." Chemosphere 216 (2019): 844-54.

Scheurer, Marco, Amandine Michel, Heinz-Jürgen Brauch, Wolfgang Ruck, and Frank Sacher.

"Occurrence and Fate of the Antidiabetic Drug Metformin and Its Metabolite Guanylurea in the Environment and during Drinking Water Treatment." Water Research 46, no. 15 (2012): 4790-802.