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Every now and then you reach into the bathroom cabinet to find that some pill or ointment you where planning to use has gone past its expiration date.

I am interested whether this expiration date has any value. In a short search I found this article from Harvard medical school stating that some test found that 90% of all drugs where still perfectly active 15(!) years after the expiration date. The article then continues to give some vague statements on possible reasons for the expiration date (including financial reasons).

What I am wondering is whether there would be any chemical reasons for drugs to become less effective or even become harmful over time, which could warrant the existence of an expiration date?

As a specific example, in case it is difficult to answer this question in general, let's take an ointment meant to treat muscle pains which use Benzydaminehydrochloride as an active component.

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It's a good question, and I cannot answer your specific example but can give some generic reasons why pharmaceuticals have an expiration date:

  • Degradation of the compound/formulation over time. This obviously comes to mind, but there is more to it than meets the eye. For example, you gave the example of an ointment and cited the active ingredient: however, the product you buy (and use) is more than just the active molecule. The whole formulation has to be stable, not only the benzydamine. The ointment is likely to be a complex emulsion, designed to target many different physical and chemical properties, including chemical stability, thermal stability, but also viscosity (so it's easy to apply), color, etc. I suppose in that particular case, the emulsion might be less stable than the active ingredient taken alone.

    Also to note: many pharmaceuticals are sensitive to air, in many cases because they are unstable in the long term in presence of oxygen, or moisture. Such products are sold enclosed in a plastic, metallic or composite packaging that protects them. There, the limited stability in time might simply be due to the limit of stability of the (cheap) packaging itself: e.g., after some time of possible light exposure, plastic will become porous and let moisture in.

  • Being conservative, because we're talking about health.

  • Being conservative, because the company selling the product doesn't want to be liable.
  • Economics: it takes money to make absolutely sure the drugs are safe for a longer period of time. Are customers willing to pay more? Most people don't store drugs for use years later (nor is it recommended), so they won't see the point in paying more for longer expiration.
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    $\begingroup$ Being cynical - to add to the economics - so that people will throw them out and buy more. The degradation point is important - if there is a drug that is supplied as a pure enantiomer, but in time undergoes racemization, the unwanted enantiomer could be toxic and/or have very serious side effects. On the other hand, some drugs can be kept for ages, but since the average consumer can't detect the degradation or non-degradation, there has to be an expiry date. $\endgroup$ – Epicentre Oct 6 '13 at 7:13
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A good question, I'd say!

As for the reason drugs 'go bad', it is true, that if the molecules are stable, they could last for a long, long time and in some cases it is perfectly possible that they would not harm anyone.

On the other hand, molecules used as medication etc. are often highly functional. This means, they contains parts that perform a reaction, bind to proteins in your body or something like that. The reason the drugs work, is this functionality. Is is, however a very good possibility for a functional group to perform a reaction that is not desired. The reason for performing the wrong reaction, is that the functional groups of the molecules are not 100% selective for their use in the body. So if any other reaction goes on with the drug, it loses/changes functionality and could even become harmful. Another term used for this process is 'degrading'. Part of the active medicine in the drug has changed into something else.

With a wide range of drugs available, it is not possible to always know which degradation product will form. Therefore, it is also not possible to ensure the drugs work, and are safe, for years and years. For health and safety reasons then, a 'best before date' or expiration date is added. This date works both ways, warns you that the drugs might be harmful, and protects the manufacturer of the drugs against misuse and lawsuits.

Hope this helped!

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