Opiates

Opiates, term originally applied to all compounds that produce insensibility to external stimuli through depression of the central nervous system, but now applied primarily to the drugs known as strong opioids—mainly compounds extracted from the opium poppy and their chemical derivatives—and also to chemical compounds that are wholly synthesized. These drugs are used to control severe, continuous pain, such as that suffered by cancer patients or accident victims, or following surgery.

The most important attribute of opiates is their capacity to decrease pain, not only by decreasing the patient’s perception of pain, but also by altering the reaction to it. Although they do have sedative properties when taken in large doses, they are not used primarily for sedation.

The major constituent of opium and the prototype of all opiate analgesics is morphine, which was isolated and chemically analysed by the German apothecary F. W. A. Setürner between 1805 and 1817. Other strong opioids used are pethidine and codeine. Heroin, synthesized from morphine, is a potent analgesic, but its use in the United Kingdom is limited to medical prescription under special restrictions.

In addition to their painkilling properties, the opiate analgesics cause a profound feeling of well-being or euphoria. When taken over a long period in large doses, they have the capacity to induce tolerance (whereby a larger and larger dose is required by the body to achieve the same effect), and ultimately psychological and physical dependence, or addiction. In this respect they are similar to barbiturates and to alcohol. However, when administered medically to control true severe continuous pain, there is little danger of addiction.

The mode of action of opiates is still not fully understood. Recent research has determined that specific regions of the brain and spinal cord have an affinity for binding opiates, and the binding sites in the brain are in the same general areas where pain centres are believed to be. This research has also succeeded in isolating compounds, called enkephalins, that are produced in the body to reduce pain; the compounds consist of five amino acids. Apparently, they can depress neurons throughout the central nervous system. They belong to a group of larger compounds called endorphins, consisting of many amino acids, that have also been isolated in the body and that are produced by the pituitary gland. Administration of endorphins, including the enkephalins, results in effects similar to those produced by opiates.

The discovery of a class of compounds that are specific antagonists to the action of the opiates has made it possible to treat opiate overdosage quickly and efficiently. The standard drug for this use is naloxone. Some of the antagonists also have opiate-like properties, and this has led to the introduction of a new class of analgesics, which includes pentazocine and nalbuphine.