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Letter Re: Some Online Resources on Determining Prescription Drug Shelf Lives
James,
I worked for 20 years as an analytical chemist and was responsible for running
experiments to determine the expiration dates of our medicines. We were always
conservative when setting limits.
There are many factors that affect the stability of medications. The most easily
controlled by the end user is temperature. As a general rule for biological
compounds near human temperatures the rate of decomposition approximately doubles with each 18° F rise in temperature
and is roughly halved with each 18° F fall in temperature. So
if the shelf life of a medicine is six months at 90° F and about three
months at 110° F. That same medicine is expected to decompose about the
same amount during four years in a refrigerator as it would in one year at
room temperature or 16 years in a freezer.
Once a package has been opened oxygen, water, or microorganisms may enter the
medicine and dramatically shorten it's shelf life. In general
