There are several reasons why it is a bad idea to share your medications. Not only is it against federal law to do so, it can also be very dangerous. The person you are sharing with may have an allergy to the medication. The drug you are sharing may interact with other medications the person is taking, including over-the-counter items. Medications also affect people in different ways, possibly leading to dangerous side effects.
People may also require a different type of drug to treat their condition. For example, respiratory symptoms may be caused by a virus, which would not be treated the same way as a bacterial infection.
Leftover drugs that are expired most likely have less effect or even dangerous effects if taken. Always remember that it is important your physician evaluate your condition before taking a prescription medication. Talk to your pharmacy to see if they have a drug take-back program, or ask how to properly dispose of your old medications to keep everyone safe. – Amy Faustino, Clinical Pharmacist