Nearly everything we use on a day-to-day basis will eventually make its way to the environment. The pills we take are no different. Our bodies only break down a proportion of the medicine we use and whatever it does not metabolise will leave our bodies and enter the sewage network. While modern wastewater treatment facilities are effective barriers for many environmentally-sensitive materials in wastewater, they cannot completely remove pharmaceuticals. Hence, after sewage treatment, some amount of the medicines we take will enter rivers and lakes as wastewater treatment plant effluent. 

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In some parts of the world, limited to no waste treatment can result in very high levels of pharmaceuticals in surface water. Similarly, waste from pharmaceutical manufacturing plants or runoff from landfill may also result in high levels of contamination in the aquatic environment.