Opiates

Opiates are made from opium, a white liquid in the poppy plant. They're also referred to as narcotics. Maybe you've heard of drugs called heroin, morphine or codeine. These are examples of opiates. Opiates can produce a quick, intense feeling of pleasure followed by a sense of well-being and a calm drowsiness. But they can also become an addiction. If someone uses opiates again and again, his or her brain is likely to become dependent on them.

Long term opiate use changes the way nerve cells in the brain work. These cells grow so used to having the opiate around that they actually need it to work normally. If opiates are taken away from dependent nerve cells, many cells become overactive. Eventually, these cells will work normally again, but in the meantime, they cause a wide range of symptoms in the brain and body. These are known as withdrawal symptoms.

Flu symptoms such as aching, fever, sweating, shaking or chills are similar to withdrawal symptoms, but withdrawal symptoms are much worse.

Source: National Institute on Drug Abuse
NIH Publication No. 97-4038