Pain is necessary to alert us to harm from injury or illness and prevent us from continually doing stupid things.
But there are a lot of situations in which pain isn’t the most helpful. Once you’re well aware your arm is broken, you definitely don’t need to be reminded of it constantly for the next 6 weeks. Similarly, once you’re immobile and drooling on the floor, you don’t need MORE pain, even if the injury technically warrants it.
Once acute pain has run its useful course, the body can take pity and release a group of neurotransmitters called “endogenous opioids” (specifically, these are called beta-endorphin, met- and leu-enkephalins, and dynorphins). Endogenous opioids can be thought of as chemical “keys” that unlock protein “locks,” called receptor sites (specifically, receptors mu, delta, and kappa).
While the receptor sites are being repeatedly activated (or, “unlocked”) by endogenous opioids, they tell the brain to feel less pain. Whoopee!!
Opioid drugs like heroin, morphine, fentanyl, oxycodone, and hydromorphone are nothing more than chemicals that are so similar to those endogenous opioids that they can activate those same receptors and have the same effect.
Unfortunately, when too many of these sites get activated at once, coma and respiratory depression can occur, causing death. While the body would be really, really hard pressed to produce enough beta-endorphin to do this, it’s actually pretty easy to ingest that much heroin.
That’s where naloxone comes into play. Naloxone is a chemical that is also beautifully shaped to fit right on those receptors…. Except that, once there, it doesn’t actually tell the brain to do anything. Think of it as a key that physically can get in a keyhole, but does not match the lock- the lock isn’t going to turn, and while it’s still in there, no other key can try.
When present, naloxone molecules attach to any free receptor sites, and every time an opioid molecule releases a site, a naloxone molecule swoops in to bind to it. Due to how fast opioids are binding and releasing receptor sites, in only a matter of seconds, most of the receptor sites can have naloxone on them.
This effectively stops an overdose, because with naloxone sitting on all the receptors, even if there’s tons of heroin molecules floating around, they can’t do anything.
Which is great for an overdose, but since the same receptors accept both endogenous opioids and opioid drugs, once naloxone is in place, it blocks both.
To answer your question, based on how it works and clinical trials that proved it works against placebo pain medication as well as opioid drugs, I’m sure naloxone could be a very effective enhancement to a torture scene, as it would prevent the character’s body from decreasing pain itself.
And it’s not too hard to get. Here in the USA, opioid abuse is at an all time high. Which means, basically (and strangely controversially), in some areas you can walk into a health department or pharmacy and walk out with a naloxone kit, no prescription needed usually for less than $20. Sometimes even for free if you say you have a spouse/partner/sibling/child/roommate who abuses opioids. Other areas you’ll need a prescription (which aren’t super difficult to get).
As for why its not used… I don’t know- maybe you’ll have to be the first writer to mention it in a torture scene!