Take a look in the cabinet or closet where you keep medications: Are any of them old or expired? If so, you should (with a few exceptions) discard them.
As discussed in a commentary in the Canadian journal CMAJ last year, people often hold on to unused medications, including opioid painkillers, because they think they may be useful in the future. But this can be unsafe if the drugs are used to self-medicate a new illness (which could delay proper diagnosis and treatment), are “diverted” to others who use them recreationally (notably teenagers in the household), or are ingested accidentally (drugs are a major source of child poisonings).
The FDA advises putting leftover medications in the trash, after mixing them with an unpalatable substance (such as used coffee grounds or cat litter) in a disposable container or sealable bag. You might also be able to dispose of them through a local medicine “take-back” program or pharmacy.
Flushing drugs down the toilet is generally not recommended because of the unknown environmental impact this may have (though far more drugs, in metabolized form, enter our water systems through human waste). Exceptions: The FDA does recommend flushing some drugs, such as narcotic painkillers and other controlled substances, because they can be especially harmful to children, pets, and anyone they are not intended for. These drugs usually come with disposal directions; the FDA has a list of them.
Of course, you may need to keep certain drugs on hand for intermittent use, such as those for allergies, sleep, and migraines. For others, notably antibiotics, you should not have leftovers if they were taken as directed.
For more information on how to dispose of unused medicines, see the FDA guidelines.