Why are supplements of nac not recommended? do they pose a threat to health? or is it just because it is not "fda approved"?

Dietary Supplements: Many are reluctant to recommend dietary supplements due to limited information (studies, data, etc.) available to them. FDA approval is a long expensive endeavor, most supplement companies lack resources for the process. Most supplements are safe in sensible doses. Drug interactions exist, and certain medical conditions can be worsened with their use. Holistic/integrative practices are expert.