Morphine is a major component of opium, a product of the poppy plant. Named after Morpheus, the Greek god of sleep, morphine is a powerful analgesic (painkiller). Doctors frequently prescribe it to relieve moderate to severe pain, especially in cancer patients. In the 1800s morphine was available in stores to anyone who wanted to buy it. At that time, little was known about physical dependence, and many people became addicted to morphine. Today, morphine is considered a controlled substance and is regulated by law. Because doctors know more about physical dependence, few patients become addicted to it.

Morphine produces a wide variety of actions, some desired and others not. The definition of a desired action and a side effect depends on the reason for using the drug. For example, opiates such as morphine can be used to treat diarrhea. However, a person taking morphine for pain would find...