The U.S. Food and Drug Administration require that all packaged foods as well as medications have expiration dates printed on the containers. The dates on most prescriptions and over the counter medications are highly conservative with medications lasting up to several years beyond the printed expiration date. In 1979 laws were passed that required drug manufacturers to publish an expiration date on their medication bottles and packaging to give consumers an idea of how long a medication would be guaranteed to be at full strength and safe. Most medications, whether over the counter or prescription, generally do not lose their potency and are not harmful if they go beyond the expiration date. There is quiet speculation that the expiration dates stamped on medication containers is actually a marketing ploy by the drug companies. Often people who see an expired date on a bottle will feel that the medication is unusable and will toss the bottle and purchase more. This creates an interesting revenue cycle.
In one study conducted by the U.S. military in conjunction with the FDA, large stockpiles of medications were tested to determine potency and 90 percent of medications tested were still at 100% potency. The FDA found that 15 year old medications stored in ideal conditions actually last years were still potent, usable, and safe. However over the counter and prescription medications stored in a medicine cabinet can rapidly degrade due to heat and humidity. If you find that pills are discolored or powdery you do need to properly discard them. If liquids have a strong smell or are filmy looking throw them away (safely). Creams that are cracked and hardened will probably be useless in curing skin conditions.
To keep your medications potent for years store in a container in a dry and relatively cool area. Do not combine medications in one bottle or the chemicals from different prescriptions or over-the-counter pills will interact and cause detrimental side effects. If you have mixed two or more medications in one container now is the time to throw them away.
Do note that insulin and antibiotics in liquid for do degrade by the expiration dates. If you have a serious health problem make sure you use all medications by the expiration date on the packaging. If you have any doubts about the activity of a particular medication you have been storing, it might be a good idea to talk to your pharmacist.