This situation is a relatively common occurrence. Many medicine cabinets are stocked with over-the-counter drugs as well as prescription medications that may be past their expiration dates. It is a good idea to routinely discard expired medicines, but if you happen to take such a drug, you will most likely suffer no ill effects.
According to the Harvard Medical School Family Health Guide, the expiration date on a medicine is not the dates when a drug becomes hazardous. Rather, it marks the period of time after which a drug company can no longer guarantee the efficacy of the medication. Since 1979, drug manufacturers selling medications in the United States have been required by the Food and Drug Administration to stamp an expiration date on their products. This is the date until which the manufacturer can still guarantee full potency of the drug.
Expiration dates also may be a marketing ploy. Francis Flaherty, a retired FDA pharmacist, has said drug manufacturers put expiration dates on products for marketing purposes rather than scientific reasons. It doesn't make financial sense to a company to have products on the shelves for years. Therefore, most drug manufacturers will not do long-term testing on products to confirm if they will be effective 10 to 15 years after manufacture.
The U.S. military has conducted their own studies with the help of the FDA. FDA researchers tested more than 100 over-the-counter and prescription drugs. Around 90 percent were proven to still be effective long past the expiration date -- some for more than 10 years. Drugs that are stored in cool, dark places have a better chance of lasting because the fillers used in the product will not separate or start to break down as they might in a warm, humid environment. Storing medicines in the refrigerator can prolong their shelf life.
Although a pharmacist cannot legally advise consumers to use medication past an expiration date, most over-the-counter pain relievers and drugs in pill form should still be fine. Certain liquid antibiotics and drugs made up of organic materials can expire faster than others. For those who still want to err on the safe side, routinely clean out medications from cabinets once they expire. However, if an expired medication is taken by mistake, there's little need to worry about potentially adverse effects.