Without looking too much into this.. FOS is not the main ingredient in the blend..so I highly doubt it's causing any harm compared to all the powerful strains. If anything it acts as a bait to confuse the yeast. Again, I believe in the bait theory. Not sure if this is really why they put it in there but I know this company is very reputable and they wouldn't be putting it in without a reason or to intentional harm people. Like I said.. the amount is miniscule and even if it is some form of sugar.. the whole dose is a quarter teaspoon and the FOS probably makes up less than 10% of that..we're talking a tiny tiny amount. If you're insanely sensitive you'll know it. For me..things keep getting better.