A narcotic is any one of a number of pain-killing drugs that are derived from opium, a resin produced by the poppy plant, or defined as any substance that behaves pharmacologically like morphine. Common medicinal narcotics are morphine and hydrocodone, which is an ingredient in Vicodin. Diacetylmorphine (heroin) is also a narcotic, but has no current medicinal use in the United States.
As the term "narcotic" is often misused in legal contexts to mean "any illegal drug", medical professionals tend to use the term opioid.
Opioids work by binding to opioid receptors in the central nervous system and gastrointestinal tract, activating them and thereby relieving pain. Opioids are some of the most effective pain relievers known and are often used for patients in severe pain.
However, opioids have several major side-effects. They are dangerous in high doses as they also suppress respiration and slow heart rate. An overdose of opioids will usually result in death from respiratory failure. The antidote to opioid overdose is an opioid antagonist like naloxone or naltrexone. They are not toxic in the sense that they cause tissue damage at high doses, and for most opioids there is no true dose ceiling. Opioids cause dependence and can be addictive, but less than 1% of patients taking them as directed develop an addiction. A sudden stop in the use of opioids in a dependent person will result in symptoms of withdrawal, such as nausea, vomitting, and increased body temperature.