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Probiotics are not just for taking on antibiotics. They do help immensely during antibiotics, though. They help with strengthening the immune system, restoring good bacteria in the gut (which is essential for a healthy immune system), regulating bowel movements, helping acid reflux, regulate yeast, help combat ear infections, and many more positive results.
I have not personally used florastor, but have heard many good things. Nordic Naturals I have used, and it's a wonderful brand. Nothing to worry about there, it's a great product.