SIDE EFFECTS OF OPIOID DRUGS

Opioids often cause constipation, nausea and vomiting. Nausea commonly occurs during labour anyway. Some individuals seem to be more susceptible to nausea than others - particularly after surgery. In labour, women often feel better once they have 'brought it up'. If it remains a nuisance, however, nausea usually responds to an antiemetic drug - given as a simple intramuscular, or intravenous, injection.

Although opioid drugs are not classified as sedatives, some degree of sedation is common. This may of course be very welcome, if it allows you to have a rest and get some sleep. The very early stages of labour - before the contractions become properly established - is often uncomfortable enough to keep you awake all night. This means that you start off in labour the next day having had no sleep at all. You will then face a possibly long and arduous day already worn out. An opioid injection is very helpful in these circumstances. It allows you to ‘drop off ’for a while before the hard work really begins.

Once labour has become established, however, not everyone finds the sedative effects of opioid drugs to their liking. Some feel 'drugged' (as if they have had a gin and tonic too many) while others sometimes feel dizzy and disoriented. Unfortunately, it is not possible to predict in advance whether an individual will find the sedative effect of opioids pleasant, or unpleasant.

All of the potent opioid drugs which are used to provide pain relief can lead to dependence, or addiction. For generations, of course, addiction has been recognised as a serious problem by society. This is why drugs of addiction are controlled by legally enforced regulations: to ensure that they are used for therapeutic purposes only. These regulations differ somewhat between different countries, and need not concern us here. The important point to stress is that there is absolutely no risk of becoming an addict after receiving an opioid drug during labour or following a caesarean section. Drug dependence only develops in those who are foolish enough to use opioids for so-called 'recreational purposes: not when they are used to alleviate acutely painful conditions - like being in labour.

Opioid drugs depress respiration: that is, they decrease the amount of air that is breathed each minute. But this needs to be put into proper perspective. After all, respiration is also depressed during normal sleep yet no one would suggest that we should stay awake all the time! The fact is, that with the kind of opioid doses used today, respiratory depression can only be detected by applying very sensitive tests. Women in labour invariably over-breathe anyway. Pain is a trigger stimulus to respiration, and far exceeds any depressant effect that an opioid drug might have. There is absolutely no fear, therefore, that an intramuscular or intravenous opioid will have any serious effect on respiration.

When given directly into the spinal fluid, however, the action of opioid drugs on respiration can be more pronounced. Then, it is possible for more serious respiratory depression to occur-sometimes (in the case of morphine) several hours afterwards. This is another reason why spinal opioids have not become more popular. Epidural opioids, on the other hand, appear to be much safer. In the author's unit, for example more than 100 000 doses of epidural pethidine have been given during the past twelve years to provide pain relief after caesarean section. During this period, no cases of respiratory depression have been recorded. Moreover, respiratory depression is easily reversed.

Opioid drugs cause the pupils to constrict, but this is perfectly harmless and hardly noticeable. Theoretical effects on the heart rate, muscle tone and body temperature are also, in practice, imperceptible. One peculiar side effect associated with some opioids is largely limited to the spinal and epidural routes of injection. This is pruritus, or itchiness. It is not known precisely why pruritus occurs, but there is no doubt that it can be a real nuisance. Pruritus seems to be more of a problem with some opioids (especially morphine) than with others. It is very rare, for example, in the case of epidural pethidine.

Contrary to popular opinion, opioid drugs have little effect on the heart and circulation. In fact, they are commonly used (in much larger doses than those used during labour) to anaesthetise patients undergoing heart surgery because they are so stable on heart function!

Finally, many people nowadays believe that they are 'allergic' to all kinds of different drugs, including opioids. Simple questioning often reveals that they have simply experienced one or more of the side effects, described above, If so, then they are not allergic to opioids at all. Allergy is not a side effect. It is a special type of reaction in which the body's immune system responds in an exaggerated way to a foreign substance. Allergy is revealed by symptoms such as skin rash, facial swelling or asthma, and the diagnosis can only be confirmed by skin testing and blood analysis. True 'allergy' to opioid drugs is extremely rare.

One of the attractions of opioid drugs is that their actions can be easily reversed by specific opioid antagonists - such as naloxone. This drug acts by displacing the opioid from the opioid receptor and so terminating its effects. Naloxone, incidentally, has been available in obstetric units throughout the world for decades. And yet, for some reason or other, popular writers (and even antenatal educators) fail to mention it! Perhaps this extraordinary omission - by people who ought to know better - explains why so many women feel alarmed about receiving an opioid drug during childbirth. From the point of view of safety, such alarm is unnecessary. The fact is that the effects of opioid drugs can be simply and quickly reversed.