One of the authors of the PPH said on Twitter "Stored properly, Dolethal (and euthanasia Nembutal in general) shelf life >20yrs". The PPeH says to store in a "cool, dark place" and that "refrigeration is fine", which leads me to believe that it will last fine without being refrigerated, as long as the original bottles are sealed and kept out of the light and in a reasonably cool location.