A lot of medical science saves a LOT of lives, and those that work in medicine fields are the real heroes of the world. Sometimes, on rare occasion, they can be used for great evil.
Then there are scientists that study pain in human subjects using capsaicin. I'm not entirely what to make of these folks.
Capsaicin is the principal ingredient in chilli peppers and pepper spray. It burns. If anyone reading this has ever accidentally ended up with even the tiniest of mild peppers in their eyes (like yours truly, not from pepper spray, but because I clapped my hands while they were covered in spicy seasoning, yes I am an idiot how did you know), I'd like to ask you the following question:
How would you like to inject some purified capsaicin directly underneath your skin?
Not even a hypothetical. Turns out, this is a fairly popular tool in the study of pain. "Pain?" I hear you say, "Capsaicin, causing pain? Ha!", and I respect your need to show your superior ability to handle the strong stuff, in which case I invite you to sign yourself up to your nearest clinical trial, because you definitely should never, ever do this at home.
Nociceptors — as in, "No no no no no" — are sensory receptors for painful stimuli. Capsaicin for instance activates TRPV1, while Menthol (often found in "heat creams", such as Deep Heat) activates TRPV8. Very cold temperatures also activate TRPV8, which probably also explains why extreme cold can "burn". The idea behind "heat creams" (topical applications of menthol, or in more severe cases, capsaicin) is that these substances when applied to the surface layer of the skin can desensitise these receptors by literally triggering those same receptors. It's a little like hitting yourself in the face to relieve the pain of a broken nose, but the weirdest thing is that it often does help. Not always, but y'know, it's a thing.
The pain response is usually extremely minimal in these topical creams because the concentrations are extremely low, combined with the fact that it takes time for it to be absorbed down through the upper layers of skin. There's also usually warnings on the box saying something like "DO NOT UNDER ANY CIRCUMSTANCES APPLY TO CUTS, SCRAPES, OR OPEN WOUNDS GENERALLY YOU DIRTY MASOCHIST", and it's for a good reason.
Don't ask me how I know.
Only a very tiny fraction of the cream is actually made of the active ingredient, but even a tiny amount will sting to high heaven when delivered directly to deeper layers of skin. Now, just try to imagine 100µg of pure, concentrated capsaicin injected underneath the skin.
Apparently, this is also a thing, specifically in scientific study, and surprisingly, it is not yet covered by the Geneva Convention as "cruel and unusual", but it does help a lot in the study of pain. For instance, one early paper I stumbled on from 1987 describes the effects of this exact test on 6 apparently-willing human test subjects, with doses ranging from 0.1µg, 1µg, 10µg, and 100µg of pure capsaicin. Sadly, unlike my previous post, there is no transcript of the conversation; it's unlikely it would have been printable. (there have been others like it, too, some more recent!)
First, there's instantaneous flaring of extreme stinging, burning pain, at a maximum intensity which lasted up to 15 minutes. For 45-60 minutes, the area all around the injection site becomes hyper-sensitive, making even the most minor sensations excruciatingly painful, which gradually wears off over time, returning somewhere to normal by the 60-minute mark, but no doubt leaving some form of mental and psychological scarring; especially if any of the participants measure their self-worth by the Scoville heat scale.
A related paper that came up while researching this topic: apparently, in 2003, topical capsaicin and menthol were both tested separately for treating a condition known as Puritis Ani, or literally: Pathologically Itchy Anus. It was found that while both Menthol and Capsaicin applied like an ointment directly to the site of itching (in this case, the butthole) caused a sharp burning sensation lasting 10-15 minutes, only capsaicin actually relieved the itching.
So yes, that's right: someone, somewhere out there, came up with this idea, and it was then tested, and the miracle of science once again raises us up to the summits, where no one's butthole is ever itchy. Though as the saying goes: no pain, no gain, even for that.
So the next time you have mates over, and that one friend starts boasting about how immune they are to chilli, definitely do not suggest that they prove themselves in this manner because we all know they're gonna do it and have all of their precious illusions shattered in the most spectacularly painful way possible, and there will be a lot of cleaning up to do afterwards.
Lord have mercy.
References:
Donald A. Simone, Jeffrey Y.F. Ngeow, Gerald J. Putterman, Robert H. LaMotte (1987). Hyperalgesia to heat after intradermal injection of capsaicin.
Brain Research, 418(1), 201-203
https://doi.org/10.1016/0006-8993(87)90982-6.
Anand P. (2003). Capsaicin and menthol in the treatment of itch and pain: recently cloned receptors provide the key.
Gut, 52(9), 1233–1235.
https://doi.org/10.1136/gut.52.9.1233
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