Antibiotic shelf life?

Curt Ashendel ashendel at
Sat Mar 11 12:19:21 EST 1995

"William C. (Bill) Johnson" <ez003195 at> writes:
>General question - how long do most antibiotics retain their effectiveness?
>Specifically - how useful is, say, the four year old tube of ampicillin I 
>found in the -20 freezer?  I've been told that most antibiotics in an 
>agar plate are only good for a few weeks, and now I wonder about our stocks.

The life of aqueous solutions of beta-lactams such as ampicillin is the 
shortest of all antibiotics and is shortend by higher temperatures and 
higher pHs. The general guideline of 6 months at -20  works only if:

A. The stocks are in water or pH7.0 buffer (if alkali is used to dissolve 
   the free acid form the solution pH can be quite high unless this is 
   done carefully.) 
B. The stocks are not thawed and refrozen repeatedly. Repeated freezing is 
   avoided by freezing in single-use aliquots.   
C. The freezer is really -20.  Our -20 freezers quickly frost up and 
   actually run about -6 to -10C  most of the time so I recommend using 
   -60C or colder freezer.

However, the time needed to make the proper solution, freeze it in 
aliquots, and the waste involved in tossing unused portions and the 
ultracold freezer space required caused me to abandon the approach using 
frozed stocks of Amp.  Ever since then we weigh out the Amp sodium salt 
(solid) and add it directly to the sterile medium whenever we need it. This 
only takes a couple of minutes and it is robust (IT ALWAYS WORKS, even in 
the hands of "lab hazards" and novices).  We have never encountered 
contamination from the addition of non-sterile solid Amp. The cons to 
this approach are that it is essential to keep humidity out of the solid 
stock bottle and that for those who may be highly allergic to pennicillins, 
this could be a somewhat dangerous task.


Curt Ashendel
Purdue University
West Lafayette, IN
ashendel at

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