"The manufacturing standards maintain that compounders impact the quality and safety of  compounded drug products. Compared to drugs compounded in outsourcing facilities or conventional manufacturing facilities, drugs  compounded in 503A compliant pharmacies are subject to less rigorous manufacturing standards and are  potentially at greater risk of product quality problems such as variability in potency and purity.  As you know, compounders that are regulated  under Section 503A of the FD&C Act include  state-licensed pharmacies, federal facilities, and  licensed physicians that have not elected to  register with FDA as outsourcing facilities.  503A pharmacies number in the thousands, and  they're primarily regulated by the states. FDA  does not inspect the vast majority of the thousands  of compounders who seek to operate under  Section 503A because these compounders are not  licensed by FDA and generally do not register their  compounding facilities with FDA.  Therefore, the agency is generally not aware of potential problems with their compounded drugs or the quality of their compounding practices  unless it receives a complaint, such as a report of  an adverse event or visible contamination. "

quoted from

• Transcript for the May 8-9, 2017 Meeting of the Pharmacy Compounding Advisory Committee (PCAC) (PDF - 954KB)