Side effects have not been reported from the use of brewer’s yeast, although allergies to it exist in some people. It is not related to Candida albicans fungus, which causes yeast infection.

Saccharomyces boulardii is registered in Europe under the name Saccharomyces cerevisiae, though the manufacturer states that S. boulardii is not the same as brewer’s yeast (S. cerevisiae). There is a case report of a person with severely impaired immune function who, after receiving treatment with S. boulardii, developed an invasive fungal infection identified as S. cerevisiae.⁸ People with severe impairment of the immune system should therefore not take S. boulardii unless supervised by a doctor.