Up to 90% of drug residues can remain in the water even after treatment.
By the way, don’t dispose of unused medications into the toilet. Besides, many residues from medications enter the water cycle through our urine – that’s why our toilets don’t mix urine and water.
Source: https://www.verbraucherservice-bayern.de/themen/umwelt/medikamentenrueckstaende-im-wasser