No, I’m not a physician. I’m not into pharmacology. But, I do know what “equianalgesic” means. I don’t usually use medical or pharmacological terms like this, but it’s necessary for what I’m about to explain.
What Does “Equianalgesic” Mean?
”Equi-“ is a prefix that means “equal.” “Analgesic” is a complete word on its own and is simply a fancy, proper way of saying “pain reliever.”
When we put these together, what do we get?
Equal pain reliever!
Okay, that doesn’t make any sense…
Anyways, the World Wide Web is home to several reliable charts that make it easy to determine equal dosages of different opioids. When combined with simple math, we can do just this.
Keep in mind that I’m not a physician. I’ve never been professionally trained in medicine. My advice is not a substitute for advice from a professional clinician.
Let’s Head on Over to Wikipedia
I encourage you to head over there now to build up some hands-on, practical experience with using this chart.
When Might Equianalgesic Conversions Come in Handy?
Physicians often use equianalgesic calculators or charts when switching patients over to new opioids for pain relief.
In my world, and likely yours, too, you may find utility from knowing how to perform equianalgesic conversions when trying out opioids that you don’t have much experience with.
Note: before going any further, know that simply using an equianalgesic chart is not sufficient if you plan on using routes of administration that aren’t covered on basic equianalgesic charts.
Take a shot for every time you read “equianalgesic chart” — doesn’t that sound like a fun game! Anyways, let’s get into crunching numbers and making sense of converting the potency of one opioid to another.
Example 1 — Converting Morphine to Hydromorphone
We’ll be assuming that all opioids will be consumed orally at first, then we’ll break out the bioavailability data. Keep in mind that most such charts — both equianalgesic and bioavailability charts for opioids — will feature slight differences. These differences are typically immaterial.
First, find “morphine.” Now, find the value in the second column. It should read 1. Now, take a visit to “Hydromorphone” Lane — you’ll see a value of 4.
Use this conversion to determine how many milligrams of morphine are equal to 32 mg of hydromorphone.
I would solve this problem by multiplying 32 by four, the “exchange rate” from hydromorphone to morphine. We’d end up with 128 mg of oral (PO) morphine — “PO,” by the way, is an abbreviation for “oral” used in medicine.
For this problem, we used this conversion: PO H/PO M.
Let’s mix things up a bit.
Example 2 — Introducing Bioavailability Into the Mix
Bioavailability refers to how well the human body absorbs drugs based on how they’re administered. Intravenous administration (IV) is always 100%. As such, other routes of administration (RoA) are expressed in proportion to IV administration.
Before going further, let’s pull out our bioavailability chart. I will be using one found on the sidebar of r/Opiates, a subreddit dedicated to — you guessed it! — all things opioid-related.
We’re dealing with IV hydromorphone, which is absorbed by our bodies entirely — aka 100%. PO (by mouth) hydromorphone’s BA is just 35%.
Let’s convert 16 milligrams of IV hydromorphone — which just might be enough for an elephant — to oral morphine. First, I would divide 16 mg IV by 0.35, resulting in roughly 45.7 mg PO hydromorphone.
Just like above, we’ll multiply this figure by four, the factor by which hydromorphone is stronger than morphine, to get 182.8 mg PO morphine.
We’ve made the following conversions: IV H/PO H then PO H/PO M.
Example 3 — Let’s Go Double on the Bioavailability Stuff
It’s time to convert 18 mg of IV hydromorphone into an equivalent dosage of intranasal (IN) morphine. First, I’d divide 18 mg IV by the oral bioavailability of hydromorphone, 0.35, to get 51.43 mg PO hydromorphone.
Using Wikipedia’s handy-dandy chart, I’d then multiply 51.43 PO hydromorphone by a factor of four to get 205.72 mg PO morphine.
Now, all we need to do is account for the intranasal bioavailability of morphine, which, according to the BA chart I’ve linked above, is between 25 and 30 percent.
Since the oral BA of morphine is already 30 percent, we don’t have to do anything to get the lower range of this answer — 205.72 mg IN morphine.
I’d multiply 205.72 mg IN morphine by 0.3 and divide the result by 0.25. What results is 246.86 mg IN morphine. Since it is greater than 205.72 mg, we can deduce that this number is the lower bound, whereas 246.86 mg is the upper bound.
Nobody really snorts morphine — the FDA found just 4% of morphine users to sniff the drug — but, this is just an example.
Again, here’s the conversions we’ve made: IV H/PO H then PO H/PO M then PO M/IN M.
Am I a Physician?
No! I’m not a clinician in any capacity. You shouldn’t be making dose adjustments by yourself.
However, unfortunately, since illicit drug users often aren’t given any assistance by clinicians, it’s useful for us to have information like this available on the Internet.
People are going to use drugs whether or not they’re offered help by healthcare or pharmacological experts — better understanding opioid equianalgesia is a solid harm reduction practice.
Do not take any information I’ve written herein as a substitute for professional advice from a licensed healthcare expert.