The Merck Manual, Section 14, Chapter 167
"Opioid (narcotic) agonists: "Opioid" is a generic term for natural or synthetic substances that bind to specific opioid receptors in the CNS, producing an agonist action.
Opioid analgesics are extremely useful in managing severe acute pain, including postoperative pain, and chronic pain, including cancer pain. They are often underused, resulting in needless pain and suffering, because the required dosage is often underestimated, their duration of action and risks of side effects are overestimated, and physicians and nurses often have unreasonable concerns about the development of addiction. Although physical dependence occurs in virtually all patients treated for chronic pain with opioids for a long time, addiction is extremely rare in patients without a history of substance abuse and should not be considered in the decision to begin or to increase doses in patients with severe pain.
Morphine, an opium alkaloid, is the prototype. In a nontolerant patient with acute pain, it provides analgesia at a dose (about 10 mg IM) that does not severely alter consciousness. Morphine affects the initial perception of pain and the emotional response to it. Patients with severe pain rarely experience euphoric sensations from morphine but may become drowsy and relaxed, partly because of decreased distress."