#KnowingYourJargon or KYJ for short is the #ect4health contribution of free open access nurse education (#FOANed). As a teacher of nurses, I get lots of questions on drugs and their side effects.
Nurse Glenda writes:
Rob, I've noticed that a drug we give at work, Targin (oxycodone/naloxone) has both an opiate and its antidote in it. Why is this? Wouldn't one cancel out the other?
Well this is a great question. The answer lies in understanding the opioid's effects and side effects.
Oxycodone, fentanyl, morphine, codeine and other opium derivatives all act as pain killers by attaching to two receptors in the central nervous system. The Mu and the Delta receptors. When activated these receptors reduce the perception of pain impulses that the patient feels. They still have the cause of their pain (Cancer, inflammation, injury) but the brain doesn't interpret it as pain. The Mu receptors specifically initiate a sense of happiness often desired as a high, or a floaty euphoric sensation, sought after by many who are opiate addicts. Heroin is a perfect example.
Now naloxone antagonises this by competing at the Mu receptor and deactivating morphine/oxycodone's ability to stimulate the sensors. Hence, naloxone is thought to be an effective "reversal" of opioid overdosage.
This brings us to the question of why you'd put naloxone in a combination treatment with an opioid like morphine or oxycodone. At the simplest reckoning, you'd think that it would stop the opioid from working but it has little effect on the pain killing effects of morphine and more inhibition of the side effects which are euphoria, drowsiness, respiratory depression, nausea and constipation.
Targin, and other combo drugs like it allow the analgesic effects but limit the negative side effects. Resp depression, and constipation are the big ones.
So how exactly does morphine and its cousins cause constipation. The bum is a long way from the central nervous system and brain, yeah?
Well here is the thing. Mu and delta receptors also line the intestines. Opiates that stimulate these in the brain, also stimulate them in the gut. When they are stimulated they have a few effects:
1- Gastric motility slows causing more transit time and dehydration of faecal matter- hence constipation.
2- strength of rectal muscles (those used to poo) is reduced, so dry faeces, is harder and more difficult for drug weakened rectal muscles to push out.
3- the nerves in the anus that detect a stool, are desensitised, causing a full rectum to be less of a stimulus to have the urge for number Twos.
So given these three issues, all leading to less moisture, weak pushing, and reduced urge, all roads lead to constipation as a major side effect of opiates.
Now let's bring back the question of adding naloxone into the analgesic... Naloxone inhibits the Mu receptor effects of constipation, yet allows the analgesic effect to still offer comfort.
If you are enjoying these #KYJs, please let me know, share this post or consider attending one of our seminars which are full of this sort of stuff.
Specifically our Rusty Pills (pharmacology refresher for nurses) is booking solidly for later this year. You need to get in and book now you you will miss out. These seminars are capped to enhance the quality and intimacy of the face to face seminars.
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