A secretin or secretin derivative protected against peptidase activity. The secretin or derivative comprises a peptidic sequence and a reactive group selected from the group consisting of succinimidyl and maleimido groups capable of reacting with an amino group, hydroxyl group or thiol group on a blood component to form a stable covalent bond. The reactive group is attached at a position along the peptidic sequence that provides, when conjugated to a blood component, a higher stability against peptidase degradation than the unconjugated secretin or derivative, and therefore an increased maintenance of the therapeutic activity compared to the unconjugated secretin or derivative. Such a compound is thus effective to provide a source of secretin having a high stability against peptidases. A method for synthesizing such a compound is also described.