Codeine, by definition, is a narcotic. It is a naturally occuring compound found in the resinous 'goo' of the Poppy Plant. The 'goo', formally known as Opium, consists of Codeine, Morphine, and various other organic compounds. The "opiates" (which is the name for all narcotic compounds derived from raw opium) are VERY powerful analgesics and cough suppressants. Their benefits are that they can severely reduce the amount of pain somebody is feeling, and give them an "ethereal" feeling. It can also control severe coughing and sedate the user as well. The drawbacks are that the opiates form very strong bonds in the brain which leads to immense addictions. These addictions are so severe that someone who routinely uses the opiates can literally die if they suddenly quit taking them. They will go into major convulsions and severe damage can result from those convulsions. Also, the opiates cause major reactions with the digestive tract of most people. While they can relieve your pain, they can also give you severe diarrhea and nausea. (When I was given codeine for my broken leg many years ago, it made me feel really good, but gave me horrible diarrhea. There's nothing worse than being on crutches; being in la-la land; and having the runs).
The benefits and drawbacks of the opium compounds are pretty much even. Thankfully, due to modern chemistry scientists have been able to develop stronger pain relievers which don't really have as many drawbacks as the raw opiates do. (Though one of these "modern marvels" turned into the worst chemical mankind has even known. In the late 1800's, doctors were concerned that their patients were all becoming addicted to morphine. Back then, morphine was a common treatment for people with a bad cough or flu-like symptoms. So chemists at the time tried to devise a form of morphine that would supress the cough, but not have all the bad side effects. They thought they succeeded when they created diacetylmorphine. Diacetylmorphine is the best cough supressant known to man, and was easy to make from raw morphine. Because of this, they called their new compound 'Heroin' since it was a hero to mankind. Sadly, there was no FDA back then so the MASSIVE addictive properties of the drug weren't discovered until it was too late. Now heroin is one of the worst things to ever grace mankind).