Flushing unused medications down the toilet isn’t safe for people or the environment. It’s better to throw old meds away. And best to “mix the medications with kitty litter or used coffee grounds and put them in ‘impermeable, nondescript containers, such as empty cans or sealable bags’ before tossing them, to help prevent accidental ingestion by children or pets. . . . [Or] add ‘some water to pills, and put some flour in liquids’ before throwing medications away.
Medications that are flushed kill helpful bacteria in septic systems. Also, drugs are still mostly intact after going through sewage treatment plants and can trickle into groundwater.