A: Morphine is a narcotic (opioid) pain reliever that is used to treat moderate to severe pain. Commonly reported side effects of morphine include constipation, lightheadedness, dizziness, sedation, nausea, vomiting, sweating, and mood changes.: Morphine is a medication that is used for pain management and is in the class of medications called opioid narcotics. Opioids act by attaching to specific proteins called opioid receptors, which are found in the brain, spinal cord, and gastrointestinal tract.