Most unused medications can be disposed of in the trash if they are mixed with an undesirable substance, such as used coffee grounds or cat litter.
Remove the medication from its original bottle. Scratch out all identifying info on the prescription label so it is unreadable.
Combine the medication with an undesirable substance and seal the concoction in a plastic bag such as a Ziploc and dispose of the bag in your regular trash.
Occasionally a prescription bottle will have specific instructions for disposal on the label. Look for those prior to doing the above.
Don’t flush unused medication down the toilet except when the prescription is specifically labeled telling you to do so. One example is the fentanyl patch, those should be flushed and the dispensing instructions will tell the user this.