Why do drugs expire?
I've googled through internet, and all I found was "what expiration dates mean". I found nothing about the underlying mechanism of expiration.
As drugs are chemicals, why do they expire? Are there chemical reactions that produce other chemicals?
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Drug molecules, even when stored safely inside a tabletcapsule, inside a closed container, upon a high shelf (hopefully), are still exposed to the environment, and are thus exposed to all of the chemical processes and reactions that go about all around us (to name just a few - oxidation, hydrolysis, isomerization, polymerization, and more). Depending on the type of drug molecule and its functional groups, the molecule may undergo all kinds of chemical processes that may change its structure andor properties.
Therefore, drugs (and foodstuffs, as mentioned in the comment above) degrade and decompose over time, and are thus given an expiration date to indicate that after a certain amount of time, the drug's manufacturer strongly recommends to avoid using the product.
An important remark should follow: the expiration date refers to the entire pharmaceutical product, i.e. the formulation, and not just the active ingredient. It is absolutely possible that the active ingredient will remain stable for a long time, but some inactive ingredient (excipient) in the tabletcapsulesyrup will have undergone some decomposition that may render the product ineffective, or even worse, toxic.
A quick Google search for "drug decomposition" or "drug degradation" yields many useful results for further reading. To name just a few:
Understanding the chemical basis of drug stability and degradation
Drug degradation (Slideshare presentation)
Pharmaceutical degradation (Slideshare presentation)
And here's a non-scientific article addressing the issue from another angle: That Drug Expiration Date May Be More Myth Than Fact (might be an interesting read for you as well)
Beside Don_S's very detailed answer I want to point out a few other things:
At least in my country, drugs are not only synthetic chemical drugs. Drugs can also be biological or contain other non synthetic chemical substances.
Examples are:
Early versions of what is now known as "Aspirin" was gained from the plant Meadowsweet.
Some preventive antivirus drugs contain weak forms of the real virus
Some of the drugs are or contain a mixture of oils and fats. They can go bad the same as for example butter after some time. Some of them have such a short life time that they are directly produced on demand in the drug store and will expire after few days or weeks.
It is hard to produce under such steril conditions and packaging that it is ensured that it will not get polluted for decades.
Another reason might be that the storage life was tested/is known for a limited amount of time and cannot be guaranteed for a longer timespan.
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