When sourcing Phenibut (4-amino-3-phenylbutyric acid), the primary benchmark is purity, which should be ≥99% as verified by HPLC (High-Performance Liquid Chromatography). Buyers must distinguish between Phenibut HCL (Hydrochloride), which is the most stable and common crystalline form, and Phenibut Free Amino Acid (FAA), which has a higher concentration of the active compound and a neutral pH. Ensure the particle size (mesh) meets your formulation needs, typically ranging from 40 to 80 mesh for standard capsules or powders.