Compositions containing a therapeutic peptide covalently linked to Vitamin B.sub.12 at the 5'-hydroxyl group of the ribose moiety of .alpha.-ligand are described. The length of the linkage is optimized so that the biological activity of both the Vitamin B.sub.12 and the therapeutic peptide is maintained. Therapeutic peptide includes insulin, PYY, NPY and GLP-1. Attachment to Vitamin B.sub.12 provides uptake of the therapeutic peptide from the digestive tract and longer residence time.