Disclosed herein are fumaryl diketopiperazine (FDKP) compositions and microparticles having a specific trans isomer content of about 45% to about 65%. The FDKP microparticles can comprise a drug such as an endocrine hormone, including, peptide, including, insulin, glucagon, parathyroid hormones and the like and can be used to make a powder formulation for pulmonary delivery of the drug.
