Relief from pain. In some people, prescription pain relievers also cause euphoria or feelings of well being by affecting the brain regions that mediate pleasure. This is why they are abused. Other effects include drowsiness, constipation and slowed breathing.
Taking a large single dose of prescription pain relievers can cause severe respiratory depression that can lead to death. Use of prescription pain relievers with other substances that depress the central nervous system, such as alcohol, antihistamines, barbiturates, benzodiazepines, or general anesthetics, increases the risk of life-threatening respiratory depression.