Hello Everyone,
Going through some of my old medications got me curious, so I found a chart online that lists relative strengths of painkillers. For example morphine is the standard and a drug like Tramadol is 10 times weaker than morphine. Whilenon the other hand a drug like oxycodene is 1.5 times stronger than morphine.
How does a scientist come up with those numbers? It can’t be subjective, I feel 10 times better on this drug than morphine. So how do they do it?
Here’s a relevant article:
They are looking at the amount of the drug it takes to produce an equal effect, not that you will feel 10 times better.
I suggest that saying Tramadol has one tenth the strength of morphine, or morphine has ten times the strength of Tramadol, makes sense. Saying something is X times weaker than something else doesn’t. It’s combining multiplying (making bigger) with lessening (making smaller).
Even that is slippery, frankly. Opioid effects are quite variable between individuals, and even within one individual given different type of pain circumstances. So all such comparisons should be taken with a BIG shaker of salt. Equianalgesic conversion tables serve only as a general guide to estimate opioid dose equivalents.
And comparing tramadol to other opioids is especially fraught with problems. Yes, it is an opioid in some of its pain-relieving action, being active at the mu opioid receptors. But other beneficial pain effects come from how it works in inhibiting serotonin and norepinephrine reuptake. It also modifies how other molecules are exchanged in the brain, in a complex chain of events that makes my head hurt.
Anyway, the basic equianalgesic doses were originally determined by giving someone with pain a certain dose of a drug (aspirin, narcotic, or whatever), and asking them how they rated it for pain relief. Then giving them a different drug for approximately the same amount of pain later, and asking them how that worked, and how well. You can see that the system is less than objective.
Since then, tweaks to the system, including modifications for how long the drug stays active in the brain, how tightly it binds to each of its particular receptor sites, how easily it’s washed out by competing drugs, etc were all employed to try to get figures on equanalgesia. It’s still more art than science. IMHO, anyway.
This, plus it’s based on feedback from hundreds, if not thousands, of test subjects.
I asked a similar question a few years back after surgery. I was given Percocet (which contains oxycodone, I think) while a friend who had the same surgery the same week (different town) was given Vicodin (hydrocodone) - and I felt my pain was well managed while she had to contact the doctor because hers was not (he added Tramadol to her Vicodin).
That led me to speculate that oxycodone was better at relieving pain, though it’s certainly possible we had different comparative dosages.
Per the Wikipedia chart both have much higher bioavailability than plain old morphine, though the oxycodone supposedly has a shorter half-life.
I’ve always wondered why they don’t just give plain old morphine, but per that chart it’s got lower bioavailability and a shorter half life. I guess that would tend to one’s wanting to repeat the dosage sooner, which is a slippery slope. I have a friend who actually IS on long-term morphine for severe, intractable back pain :(.