I have Retin-A cream. The exp date says Dec 17. Is the 17 the day or the year it expires?

Properly stored, most medications can even be used years after their expiration dates. Expiration dates on medications are pretty much an arbitrary date stamped on the medication by its manufacturer, likely to create planned obsolescence and spur the need for repeat purchases of its product.  Frankly, it has been known for many years by them and this applies to most topical medications (including eye drops) and oral medications (including antibiotics), that most medications are still effective many years past their expiration dates, so long as they have been properly stored. The U.S. government conducted a study once that strongly supported this, having found that well more than a decade following their expiration date, even oral antibiotics retained about 80% of their original efficacy--enough to do the job. Properly stored ironically means not in the bathroom medicine cabinet that is exposed to constant year-round heat and humidity. And medications should not be exposed to light. Of course, if for any reason, any medication, topical or oral, is discolored or the tablets somewhat disintegrated, it should be discarded. One notable exception to the above is sunscreens. If never opened the product may still be good years later. However, once opened, they are good for that sun season only and should be tossed at the end of the season. So, the short answer to your question is that, so long as you have stored your Retin-A cream in accordance with the above, you can still use it way past the December expiration date. All the best.  

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Typically it's the month and year of expiration on the box, but if you are still unsure I would call the pharmacy for confirmation.