When sourcing Phenibut (4-amino-3-phenylbutyric acid), the purity level is the most critical metric, typically requiring a minimum of 99% purity as verified by HPLC (High-Performance Liquid Chromatography). Buyers should distinguish between Phenibut HCL (Hydrochloride), which is the most common stable crystalline form, and Phenibut FAA (Free Amino Acid), which has a higher concentration of the active compound per gram and a neutral pH. Ensure the product meets USP (United States Pharmacopeia) or EP (European Pharmacopoeia) standards where applicable, and always request a COA (Certificate of Analysis) from the specific production batch.