They act by binding to GABA A receptors thus decreasing the sensitivity of the postsynaptic neurons. These are substances isolated from the opium poppy or synthetic relatives. This explains why opiates are such effective pain killers. Opiates also inhibit brain centers controlling coughing, breathing, and intestinal motility. Both morphine and codeine are used as pain killers, and codeine is also used in cough medicine.
Opiates are exceedingly addictive, quickly producing tolerance and dependence.
Although heroin is even more effective as a painkiller than morphine and codeine, it is so highly addictive that its use is illegal. These are located on the subsynaptic membrane of neurons involved in the transmission of pain signals. By binding to enkephalin receptors, opiates like morphine enhance the pain-killing effects of enkephalin neurons. Opiate tolerance can be explained, at least in part, as a homeostatic response that reduces the sensitivity of the system to compensate for continued exposure to high levels of morphine or heroin. When the drug is stopped, the system is no longer as sensitive to the soothing effects of the enkephalin neurons and the pain of withdrawal is produced.