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heteromer t1_ivo4i35 wrote

Drugs when applied topically can absorb systemically. For products like Aspercreme, however, they only need localised effect, so they don't need to get into systemic circulation in appreciable amounts. They just need to cross through the skin and into the local area (i.e., muscle). They passively absorb through the skin via two ways:

  1. transcellularly, where they cross through the cell membrane and through the skin cells, or

  2. paracellularly, where they cross in junctions between cells.

For the latter, there are excipients like ethanol or glycerin (which is found in Aspercreme) that can help open up these junctions to facilitate paracellular diffusion through the epidermis. Thick ointment formulations also help in the same way. The active drug, salicylate, won't help with nerve pain but it can provide a localised anti-inflammatory effect -- the advantage with localised effect is the drug compound itself doesn't need to be so potent. Nonetheless, the active drug still has to have some favourable physicochemical properties -- it has to be highly lipophilic, but still sufficiently hydrophilic, to cross transcellularly, and it needs to have a small molecular weight to fit through functions, etc. The use of a triethanolamine salt for salicylic acid also helps, as it helps to stabilise the drug (think of it as the rocket that propels the shuttle before detaching). Triethanolamine salicylate has been shown to perfuse tissue.

Interestingly, methylsalicylate is another form of salicylic acid used in topical preparation. There are enzymes called esterases in the skin that will cleave up molecules by their ester linkages. Methylsalicylate contains an ester group that gets hydrolysed by these esterases when absorbing, turning into the active salicylic acid.

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Optimal-Conclusion t1_ivpjsk9 wrote

So my body is a big ole' sponge? What else gets absorbed versus not? Should I be wearing gloves more often than I thought?

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Zenmedic t1_ivrbm6n wrote

There's a fairly long list of things that can be absorbed through the skin. LSD and THC can both be absorbed, however there is limited systemic intake, so you need a lot, or the right conditions. This is the same for many things.

This is why as a former HazMat Medic, I would always rinse people with cold water. Close the pores, decrease circulation (not because I'm a sadist).

Warm conditions, certain carriers (as mentioned above) as well as general skin condition and thickness of epidermal layer also play a big role.

I was always taught that if you don't know if you should be wearing gloves/suit/respirator....the answer should default to yes.

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Sharlinator t1_ivsmom8 wrote

Well, I'm sure you have experienced how something like hand lotion is absorbed quite effectively (and indeed that's its whole purpose)!

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heteromer t1_ivsrbz2 wrote

The epidermis is highly keratinised to prevent systemic absorption. Drugs that absorb systemically in concentrations high enough to exert an effect are quite rare, and the epidermis does quite a good job as a barrier.

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provert t1_ivoyfha wrote

Excellent and thorough! Thank you!!!

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freakydeku OP t1_ivr1bk4 wrote

thank you! i appreciate you going into detail on the function. i don’t yet fully understand all the terms you’re using but i’m glad you used them so i can have a better/wider/ more holistic understanding

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greenappletree t1_ivvfsx6 wrote

A bit of a tangent. This is a reason to be carful when using “topical” since it can be absorb and overdose is possible. There has been cases where people have overdosed on simple analgesics for example.

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xiledone t1_ivo0uta wrote

Are u asking about mechanism of action? Because it's the same as most nsaids.

If ur asking about absorption, topical application still gets absorbed. Your skin (keratin mainly) is hydrophobic, so it resists water and other charged substances from entering, but there is a hefty list of chemicals that, if touched, can be absorbed into your bloodstream and even be toxic (notable, methylmercury, as only a tiny bit is touched is needed to be lethal, the methyl means it has extra carbon and makes it more like an oil and bypasses the skin's hydrophobic effects easier).

Hydrophobic stuff will resist charged particles, such as water, from interacting, but will allow other hydrophobic stuff, such as oils and anything thats mostly carbon, to interact. For instance, pure lead (a metal, so its charged) isn't that toxic unless u inhale or ingest it, as the skin works well to resist it. However, tetraethyllead (lead with extra carbons) is very toxic via touch.

(Small edit for clarity, oil actually doesn't get absorbed that easily, as its a big molecule, but its the most easily recognizable hydrophobic liquid, so it was used as an example to make the concept easier to grasp. Just wanted to clear that up).

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ZSpectre t1_ivo3vfb wrote

Ah, tetraethyllead. The anti-knocking agent used in gasoline that caused the whole world to get dumber. And hearing this explained while knowing how the inventor of leaded gasoline straight up inhaled the stuff for a full minute in front of the press in order to ease the public's concern of toxicity (his health didn't fare very well afterward).

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SharkLaunch t1_ivo7nll wrote

Don't worry, it definitely didn't affect him when he went to go work for Frigidaire. Nothing important, he didn't break anything, please ignore the hole in the Ozone layer.

(Anyone who's confused, please Google "Thomas Midgley Jr" to learn why many of our problems are his fault)

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ZSpectre t1_ivoc9eu wrote

Yeah, to those passing by, I recommend the video by veritasium that I think was titled something along the lines of " the man that accidentally killed the most people in history." And I just looked up the timeline and it turns out that you were correct on how he was still mentally functional enough after his stunt inhaling the gas (1924) in order to streamline and mass market freon / CFCs in 1928.

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[deleted] t1_ivo7lty wrote

[removed]

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open_door_policy t1_ivohx8z wrote

It's a valid theory. Especially when you realize that it gets released back into the blood as old people shrink.

So as they've gotten older, all the paint chips they ate and the gasoline they smelled is probably coming back to bite us.

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Saint_Oliver t1_ivo2qzk wrote

When molecules are absorbed, do they go into cells or in between cells (eventually to the blood stream I suppose?)

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xiledone t1_ivo3jix wrote

Both, but heavily depends on the molecule and the cell. The question is more nuanced, so unfortunately so is the answer, cuz the body is so complex there's rarely a straightfoward answer.

But if allowed to overgeneralize, most of it goes inbetween and "through" the wall that the cells make.

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freakydeku OP t1_ivr14ym wrote

thank you! yes i was wondering about absorption. i’ve had really bad peroneal nerve pain for a couple years that i’ve had trouble getting relief from. i never used topical ointments because it feels like a “deep” pain and i assumed it wouldn’t work but recently i tried and it did. i found it a bit confusing so i was wondering how it affected my nerve

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[deleted] t1_ivo5u15 wrote

[removed]

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cantcountnoaccount t1_ivpb3t7 wrote

Capasin and menthol ointments are counter irritants. Some of them contain lidocaine, a topical anesthetic (makes your skin feel numb)

Some lineaments and rubs don’t contain a counter irritant. Rather, the action of rubbing it in helps your body remove inflammation or helps loosen a tense muscle. Inflammation causes part of the pain you feel.

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