There are two major contributors to deterioration of medications – heat and moisture. OK, so not a big deal – right? How about expiration dates — how important is that?
Consider this – many people keep their medications in the bathroom medicine cabinet (surely it must be called that for a good reason!) Often positioned above the sink and in the same small room as the shower or bath, the medicine cabinet is hardly the ideal environment for oral drugs that are not individually sealed against moisture. Consequentially, Paramedics often bring in bottles of “clumped” medications they find in the medicine cabinet along with the patient to the hospital. (This is also a good indication the patient was not taking their medication).
But there is more – Heat and moisture also contribute to the breakdown of medications. For example, you may be greeted with good whiff of vinegar from aspirin tablets that have been in your medicine cabinet for a month or two. What you smell is breakdown of aspirin (chemically known as acetylsalicylic acid) into acetic acid by moisture. Vinegar (acetic acid) is useful on a fresh salad but it won’t do much to treat pain, fever or help prevent a heart attack.
It is important to keep your medications in a cool place, out of direct sunlight, and at 
reasonable humidity. In high humidity locations, place your individual medication containers in moisture-proof sealing containers or zip-lock bags. Obtain a fresh supply of medicine if you suspect heat or moisture may have compromised the effectiveness of your drugs. Also, if you have not used a prescription drug for the better part of a year, you may be best served by properly disposing of it (particularly if kept in a bathroom medicine cabinet).
A bit about expiration dates – To maximize safety and effectiveness, the Food and Drug Administration (FDA) initially required expiration dates on prescription and over-the-counter drugs in the 1970s. See the video.
Expiration dates for medications are determined by the manufacturer and must assure that as of that date the drug has between 90 and 110 of the labelled potency when stored under ideal conditions (unopened bottle, within heat, light and humidity guidelines).
While some medications, such as reconstituted suspensions, and certain injectables (ex. epinephrine) are not stable for extended periods, that may not be true for many tablet type of drugs. Some studies demonstrate that many oral solid dosage form (tablets, capsules) prescription drugs are within the 90-110% potency range years after their labelled expiration date has passed. So, why aren’t the expiration dates extended?
Keep in mind the manufacturer is not required to determine the maximum time of drug stability for the expiration date—just a date that it knows the drug will be in the required 90-110% potency range (under ideal conditions!) And yes, a shorter expiration date is likely to result in more sales to pharmacies to replace drugs nearing expiration dates.
Also, many states are now require a note on the label that unused prescription drugs should be discarded one year after it is dispensed to the patient. This is intended to discourage patients from keeping prescription drugs for extended periods as once the manufacturer bottle is opened and the drug dispensed to the patient, there is no longer compliance with FDA “ideal” storage requirements. Therefore, the manufacturer expiration date is no longer relevant.
There aren’t many (if any) studies of how long your prescription drugs are stable and 
effective when subjected to and stored in the less than ideal conditions associated with daily life. If you have kept a prescription drug for more than a year, it is probably a good sign it is not needed, may be less effective than intended, and may adversely interact with newer prescription medications you have recently started taking. For that reason, many states have implemented guidelines requiring prescriptions are valid only for a year and must be renewed annually.
Last thoughts. The (FDA) requires expiration dating on prescription and over-the-counter medications but this does not apply to homeopathic, herbals, and dietary supplements. The major reason for this seems to be the FDA believes that any active ingredient is likely to be diluted, not properly identified, or cannot be easily measured in a consistent manner. Any expiration date on these products may be arbitrary, not valid or reliable and is not endorsed by FDA standards.
Overall, expiration dates allow consumers to have confidence that the prescription or over-the-counter drug will have the labelled potency up to that date. The conservative nature of expiration dates reflects the FDA focus on patient safety.
Read more:
Diven, D.G., Bartenstein, D. W., AB, Carroll, D. R., (2016). Extending shelf life just makes sense. Mayo Clinic Proceedings. Nov 2015, Vol 90, No 11: 1471-1474 from http://www.mayoclinicproceedings.org/article/S0025-6196(15)00667-9/pdf
Science Based Medicine: www.sciencebasedmedicine.org/the-drug-expiry-date-a-necessary-safety-measure-or-yet-another-big-pharma-conspiracy.
United States Food and Drug Administration and Department of Defense Shelf-Life Extension Program of Pharmaceutical Products: Progress and Promise. Journal of Pharmaceutical Sciences May 2014, Vol 103, No 5: 1331-1336
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