Some specialty brands of peanut butter have begun adding xylitol to their low sugar products. None of the main manufacturers are doing this yet.
Xylitol is commonly found in sugar free gum and can be dangerous or even fatal to dogs who eat enough of it.
Please make sure and read food labels carefully before feeding to your pets, especially low and no-sugar products. Many people treat their dogs to peanut and other nut butters or use them to administer medications.
See related article:
http://www.preventivevet.com/dogs/is-peanut-butter-safe-for-dogs