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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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    Chemicals and food break down. – paparazzo Nov 15 '17 at 11:57
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    This was already asked at the Chemistry.SE (not that there's anything wrong with asking it here, I thought I'd just point it out) ;-) – paracetamol Nov 15 '17 at 15:59
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    All things that expire are chemicals. Further more all things are chemicals.Being chemicals or not has no bearing on whether or not things expire. (well not being chemicals means it doesn't exist so renders any question of expiration null) – Lyndon White Nov 16 '17 at 1:55
  • @LyndonWhite - energy exists, but I wouldn't necessarily call it a "chemical". Some forms of it come from chemical reactions, but some don't. And a lonely photon flying through the universe isn't really a chemical in my view. – Davor Nov 16 '17 at 10:10
  • Energy (including photons) are not "things" in my mind. (though my bracketted section isn;t clear) – Lyndon White Nov 16 '17 at 13:26
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Drug molecules, even when stored safely inside a tablet\capsule, 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 and\or 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 tablet\capsule\syrup 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)

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    Thank you so much. Why would a drug have inactive ingredient? – Asmani Nov 15 '17 at 14:09
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    @Asmani To make sure the active ingredient gets where it should (capsules), to decrease concentration of active ingredient, to make the patient tolerate the drug. You could go ahead and ask that as a new question, it is a good one. – Narusan Nov 15 '17 at 14:25
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    "The non-scientific article" is actually a well researched joy to read. I wanted to add a tidbit – but it's already in there. You might add more from that. // This expiration date is indeed an indicator of well meant and necessary regulation that's gone bad itself, leading to massive waste. – LangLangC Nov 15 '17 at 14:35
  • What do they add in drug tablets when the amount of the drug is too small, like 1 mg Clonazepam? – Asmani Nov 15 '17 at 18:10
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    @LangLangC That's true. For prescription drugs, it's often the prescription that expires before the drug itself actually expires and this can cause legal complications. For example, where I live, if you have some prescription pain killers left over (say, from a minor surgery when you didn't end up needing to take all of them,) if you have some other need for them come up later, it's illegal to take them after the prescription expiration, even though the drugs themselves may still be fine. – reirab Nov 15 '17 at 20:54
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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:

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.

  • Welcome to HealthSE! This looks like a very interesting answer. Please take the tour and read the help-docs to get to know ths site. While this comment doesn't judge your answer (imho it's going somewhere)please be aware of the site policy to provide reliable references to backup an answer, so that anyone can check the validity of it. – LangLangC Nov 15 '17 at 23:04
  • @LangLangC I am sorry I can't provide better sources than Wikipedia because I have my knowledge from non-English printed medical/chemical books and course-material in the laboratory I worked. – H. Idden Nov 18 '17 at 0:05

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