As well as killing pain, moderate doses of pure opioids produce a range of mild effects. They depress the activity of the nervous system, including such reflexes as coughing, breathing and heart rate. They also cause widening of the blood vessels, which gives a feeling of warmth and reduces bowel activity, which causes constipation.
Even with doses sufficiently high to produce euphoria, there is little change to co-ordination, sensation or intellect. At higher doses however, sedation takes over and an overdose will result in unconsciousness, coma and very rarely death from respiratory failure. The chance of an overdose is greatly increased if other depressant drugs such as alcohol or tranquillisers are being used at the same time.

Opioids cause people to feel drowsy, warm and content. They also relieve stress and discomfort by creating a relaxed detachment from pain, desires and activity. However, once users have developed physical dependence and tolerance (needing more and more to get the same effect) to opioids, positive pleasure is replaced by relief at simply taking the drug to maintain 'normality'.

Codeine is extracted from the same poppy flowers that also yield heroin and is a addictive analgesic drug.