A method is provided for purifying a peptide or a protein comprising: providing a peptide or protein composition containing an impurity to be removed forming a complex of the active agent with a complexing agent thereby displacing the impurity and, removing essentially all of the impurity from the complex. A composition is provided comprising such a purified peptide complexed with a complexing agent. Microparticles comprising monomeric or dimeric insulin complexed with a complexing agent, wherein the insulin is substantially free of zinc, may be used in medicine to deliver insulin to a patient. Also provided is a formulation for use in medicine comprising insulin complexed with a complexing agent which, when administered to a patient in an effective amount, produces a peak blood insulin concentration in the patient that simulates a normal response to eating, wherein the formulation is suitable for non-intravenous administration.
