An absorption promoter is disclosed which interacts with hydrophobic amino acid residues of polypeptide derivative medications and substantially increases the absorption of that medication administered orally or rectally. The promoter and medication can be compounded into unit dosages in capsule, tablet, or suppository form, or combined into elixir solutions, suspensions, etc. The absorption promoters are phenylalanine derivatives which may be prepared through conventional N-acylation techniques.