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There is no simple answer to this question.
It depends on the circumstances for which they're being used. It's likely that you will need different probiotics following gastrointestinal infections, with the use of antibiotics, with irritable bowel
, leaky gut or inflammatory bowel, etc. See http://www.Klaire.Com/probiotics_cat.Htm for a list of options. A good basic option is iflora, well balanced.