HOW OPIOID DRUGS WORK

Opioids mimic the effects of ‘morphine-like’ substances which are normally produced by the body, called endorphins. Endorphins are produced at various sites in the body and function presumably as the body’s natural defence against pain. Endorphins act by attaching themselves to nerve cells (neurons) and depressing their activity. They attach themselves at specific sites on the outside of the neuron which we have come to refer to as opioid receptors. After they ‘occupy’ the receptor they stimulate a chain of reactions which results in a depression of their normal activity for a short time. They then leave the receptor and the normal function of the neuron returns. Opioid receptors are found in large concentrations in the brain and spinal cord which helps to explain why the effects of opioids drugs vary with their route of administration.

We are not usually aware of the effects of endorphins on us, however, they are said to be secreted in higher amounts after exercise and orgasm, hence the feeling of well being which can follow these events. There is some evidence that they are secreted in larger amounts just before labour commences and may act to temporally increase a woman’s pain threshold during this period. ‘Natural techniques’ like TENS and acupuncture are felt to work at least in part by stimulating the production of endorphins in the spinal cord.

Opioid drugs, on the other hand, are more potent and their effects are readily seen. They have the same depressant effects as endorphins because they look similar and are able to fit into the opioid receptor without it realising that it is not endorphin. When this occurs on a nerve which is responsible for transmitting information about pain from the site where the painful event is coming from, to the brain, then the effect will be pain relief! When it occurs on other neurones, then the effect will be to depress whatever the normal function of that neuron is.

This effect might be to stimulate breathing, or to stimulate the bowel to contract. Hence the mechanism by which side effects of opioids like respiratory depression and constipation occur. These effects are dose related - the higher the dose given - the stronger the effect. This holds for both pain relief and side effects. The only potentially dangerous side effect of opioids is severe respiratory depression. Mild degrees of respiratory depression are not dangerous. Opioids can be effective against moderately severe pain in doses which do not cause dangerous degrees of respiratory depression. When pain is not relieved by recommended doses then alternative forms of pain relief should be used rather than increasing the dose.