Just five days of exposure was all it took to show that morphine treatment in rats caused chronic pain that lasted for months. The morphine triggered a cascade of pain signals from immune cells in the spinal cord, which normally are meant to clear out infection-causing microorganisms. This cascade of signals increases the activity of nerve cells, which in turn leads to a long-lasting chronic pain. If the study holds true for humans, it could be a major step in explaining the outbreak of prescription painkiller addiction in recent years. Those who have taken opioids such as morphine, oxycodone and methadone may have actually been doing more harm than good as these drugs may simply numb an issue that they concurrently make worse.