What does it mean to say one opioid is X times stronger than another? For example, I read carfentanil is 10,000 times stronger than morphine while tramadol is about 1/10 as strong as morphine. This seems to say we could fix any amount of pain by taking a big enough dose of any opioid, though you’d need 100,000 times as much tramadol compared to taking carfentanil.
That’s not correct, is it?
You can read a bit more here: Equianalgesic - Wikipedia.
Drugs also have side effects. Taking a 10,000x dose of morphine might give the same pain relief as a 1x dose of carfentanil. But it’ll also give you 10,000x the side effects. Whhich might be unpleasant.
Death be not pleasant.
It means it attaches to the receptor more strongly. Morphine may attach to the opioid receptors, then release, then reattach later. Carfentanil is like super glue and stays attached to the receptor once it finds it, and doesn’t let go, causing the protein mechanisms to keep running.
The human body has roughly 40 trillion human cells in it (and another 40 trillion bacteria, but they don’t have opioid receptors as far as I know). A 200mcg dose of fentanyl has about 300 quadrillion molecules of fentanyl, so about 7500 molecules of fentanyl for every cell in your body
Even carfentanil at a lethal dose of 20mcg still has 30 quadrillion molecules. I’m not sure how many opioid receptors are in the body, but thats still about 750 molecules of carfentanil for every cell in your body. Once it finds a receptor and binds, it doesnt let go.
There is also the fact that some drugs are full agonist and some are partial agonists. Tramadol is a weaker receptor agonist.