Explanation for potency of repeated oral doses of morphine?

Morphine given by injection is the standard by which all other strong analgesics are measured, but when given orally in single doses it is a poor analgesic. With repeated oral administration it becomes very effective and it may be that on repeated dosage active metabolites, particularly morphine-6-glucuronide, account for much of the analgesic activity. Enterohepatic circulation may also contribute to the maintenance of blood and tissue levels of morphine and its metabolites in chronic use.