Sources: U.S. Food and Drug Administration (FDA) Compounding and the FDA (503A & 503B guidance) FDA warning letters on unapproved peptide products National Institutes of Health (NIH) PubMed FDA Sends 30 Warning Letters To Continue Clampdown on Compounded GLP 1s BioSpace The FDA has issued warning letters to multiple companies marketing compounded Retatrutide, an investigational drug from Eli Lilly still in Phase II trials. While early studies have shown promising weight loss results, Retatrutide FDA Warning Letter Breakdown: Sterility Failures, Subpotent GLP 1 Drugs, and a Compounding Pharmacy Operating Without Control