Many probiotic bacteria are naturally sensitive to heat and moisture. Heat can kill organisms and moisture can activate them within pills, only to die due to lack of nutrients and a proper environment. These products should ideally be refrigerated and kept out of humidity. However, probiotics with freeze-dried organisms (which includes most sold as supplements in tablet or capsule form) and in packaging to prevent moisture, such as blister packs, generally do not need refrigeration or to be kept out of humidity but should still not be exposed to heat above room temperature. They also have longer shelf-lives than products containing live cultures, such as yogurts and drinks (which must be refrigerated). Probiotic yeast and some of the spore-forming bacteria, such as Bacillus coagulans, generally do not require refrigeration.
If you are purchasing a probiotic with a label that suggests or requires it be refrigerated, be sure your retailer has kept it refrigerated. If you order the product by mail, such as from an online retailer, be sure it is shipped overnight or with refrigerated shipping to minimize exposure to extreme heat, especially during warm weather - and arrange for the package to be delivered at a time you are home. (Freeze-dried probiotics are not damaged by extremely cold temperatures, like the temperature in airplane cargo holds, which can reach freezing or below - so they can be safely shipped by air.)
Once you get the product, be sure to follow the manufacturer's recommendations. If in doubt, refrigerate. Once you remove a probiotic from its blister pack or container, use it right away - don't put it in a weekly pill holder.
For more information about probiotics and ConsumerLab.com's latest tests and comparisons, see the Product Review of Probiotic Supplements.