Florastor is a probiotic made of live freeze dried yeast cells of the species Saccharomyces boulardii. The cells are put into capsules and sealed. Each capsule of Florastor has 250 mg of S. boulardii in it. Probiotics help the good organisms in your body to grow and protect your intestines against "bad" organisms that can make you unwell. It is 1 of 3 probiotics I took to treat c.diff.