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This is something I have read on forums repeatedly, but I don't understand the mechanism behind how antibiotics lose their effectiveness. I understand how repeatedly thawing and freezing cells would damage them, but not how repeatedly thawing and freezing antibiotics make them lose effectiveness.

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    $\begingroup$ do you have any sources for that? $\endgroup$ – pH13 - Yet another Philipp Aug 18 '15 at 13:32
  • $\begingroup$ protocol-online.org/biology-forums-2/posts/9077.html - It does not answer if repeated freezing and thawing actually affect antibiotics performances. $\endgroup$ – wswr Aug 18 '15 at 15:18
  • $\begingroup$ If you are talking about dry pills, I doubt if keeping them in the freezer would do any harm. Liquid formulations might use emulsifiers to keep antibiotics in a stable suspension, and freezing/thawing could break up the colloid and cause the ingredients to separate. $\endgroup$ – iad22agp Aug 19 '15 at 6:15
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As indicated in the comments, freeze-thaw cycles should not affect molecules of size of typical antibiotics. Freeeze-thawing may affect colloids by disrupting their structure. Also, the effect would depend on how fast a sample was frozen/thawed etc (slow freezing would cause rearrangement of molecules and crystallization of water whereas rapid freezing would result in vitrification of water.)

The popular lab practice, in my opinion, would have perhaps arisen because it is better to keep the stock stored at the appropriate temperature (frozen) for longer life. Repeated thawing of the stock would definitely increase its chance of degradation because of mishandling and even otherwise. Therefore, it is better to aliquot smaller volumes; even if you have forgotten to keep it back into the freezer you'll end up losing only a small amount.


Not related to your question but this is an interesting fact: freeze-thaw cycles promote molecular assembly of RNA into higher order structures (ref).

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  • $\begingroup$ Nice job here WYSIWYG. The premise of the question is just another one of those hoary molecular biology / microbiology lab myths that refuse to die... $\endgroup$ – Curt F. Sep 21 '15 at 12:49

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