Opioids are among the oldest and most enduring substances used for the management of pain and other medical problems. It is now known that the properties of the naturally occurring opioids (primarily morphine and codeine) and the related endogenous opioid-like peptides and synthetic chemical substances can be attributed to action at the receptors to which they bind within the central and peripheral nervous systems. This book presents current research in the study of the pharmacology, clinical uses and adverse effects of opioids. Topics include an historic review of the pain concepts and treatment by opioids; u-opioid receptor modulation of oxytocin and vasopressin release in the hypothalmic-neurohupophysial system and synergy abilities of spinal opioid receptors.