Review (4/5): Many probiotic side effects are due to release and growth too early in the gut.
Personal experience that may or may not be helpful.
If you have IBS, you may well have small intestinal bacterial overgrowth (SIBO). In either case, taking a probiotic that is not enteric-coated or coated for release AFTER your small intestine may make your symptoms worse because you are recolonizing your small intestine (this includes Align, and most on the market).
This product is enteric-coated, so should release after the small intestines to avoid putting bugs in the wrong place. Read product info carefully to confirm enteric-coated, since they have other products. They also have liquid versions that will dump the bugs in your small intestine.
If you're microbiome is jacked up, BIG doses of any probiotic may mess you up. I used the 12.5 billion version of this product (available elsewhere) to avoid over-doing it and it seemed OK for me, but maybe not you (see other review on backing off 25 billion). Take home message is always start small. Tread lightly.
If you react, this may be rapid growth in your small intestine with bloating.
If you react, you may be reacting to the prebiotics in the product, not the probiotic itself. Avoid prebiotics initially if very sensitive.
These bacteria may release d-lactate (brain fog).
They may make you feel worse regardless particularly if you're sensitive to histamine in food, since these strains make histamine as a byproduct of normal metabolism.
Bifidobacteria may not do this to the same degree, but these again should be engineered for release later in the gut (maybe Pearls Adult 50+ or others). Pearls Adult 50+ contain only bifidobacteria similar to those in Align. Align dumps in your small intestine.
S. boulardii is a yeast and seems to be more tolerable (to me) that may help repair and get combat bad bacteria, but it will not of itself rebalance your bacteria.