Where does it hurt?
The World Health Organization delineates a myriad of ways in which leftover medications are treated, many of which do not protect the aquifers and watercourses. The most effective method, high temperature incineration, is complicated and expensive. Medications can also be immobilized by encapsulation or inertization, or they can be disposed in landfills, processes which do not prevent leaching.
Although not a priority with Big Pharma, the nascent field of sustainable pharmacy presents some hope for the future. It considers factors like the designs of compounds, their packaging, manufacture, prescribing and disposal; in short, the entire lifecycle of a drug in an effort to reduce risks to wildlife and the environment.