All of the packages of TM that I buy are sealed foil, to keep out
moisture and UV light.  On the package, it states:  Store below 25 degrees
C or 77 degrees F.  I have been told that TM breaks down with
moisture, sunlight, and heat.  The storage warning certainly implies heat
sensitivity, and many of our veterinarians store it under refrigeration.
 
Jerry Bromenshenk
The University of Montana