How does topical medicine get into your bloodstream? If I spill apple juice on myself does apple juice enter my bloodstream?
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Your skin is somewhat permeable and you will absorb some chemicals through it, though the amount is generally very low. Think of using a cheese cloth to separate fruit juice from the seeds, or how water can drip through a coffee filter but the grounds will not.
So yes, if you spill apple juice, parts of it will enter your bloodstream.
Two things:
1. You know how oil and water don’t mix, but if you combine two water-based liquids (like water and fruit juice or something), they do mix together? Chemists call that “like dissolves like”. Basically, fatty things can mix with each other, and water-based things can mix with each other, but fat-water combos can’t mix.
2. Your cells all have outer membranes (skins) that are mostly made of fat. Including your skin cells. That means your skin presents a fatty surface to the outside world.
The result of those two things is that (like DragonFire said) your skin is somewhat permeable…but especially to fats and oils, and much less permeable to water. That’s on purpose, to prevent your body losing a ton of water to evaporation. That’s also why you don’t inflate when you go swimming, and why you can spill apple juice on your skin and essentially none of it absorbs.
BUT with fats it’s a different story. You’ll notice that all of the topical medicines are in a greasy or creamy suspension. That’s also on purpose, they’re fat-based and water-free so that they can “mix with” and absorb into the fatty membranes of your skin’s outer layers and into the tissue beneath. Polysporin, Vick’s vapo-rub, BenGay, they’re all greasy. Even Vaseline and baby oil absorb into your skin for the same reason.
Related fun fact: An organic solvent called DMSO (dimethyl sulfoxide) is gasoline-like (fat soluble) and smells sulfur-y. If you dip your finger into a beaker of DMSO, you can almost immediately taste garlicky in your mouth because it rapidly absorbs into your blood and is exhaled in your breath.
If it’s pure DMSO this is pretty harmless, but chemists have to be careful working with this solvent. Anything else already dissolved in the DMSO will be rapidly carried into the body along with the DMSO as it rapid- absorbs into the skin.
There are two reasons to really use a topical formulation. one is you don’t want the medicine in your bloodstream. two is you want very slow, controlled absorption. Let’s start with the first one.
One reason you would choose a topical formulation of medication is so you can get effects right where you want them without the medicine getting everywhere in your body and causing side effects.
For example, say you have something irritating and itchy on your leg, a little bit of poison ivy, or a mosquito bite, you might feel like you want to use a steroid to reduce the itchy feeling. Now if it is really bad (like a severe allergic reaction), you need to go to the doctor to get an oral steroid or even something injected to make the symptoms go away. The problem is that causes a lot of side effects, blood sugar issues, hot flashes, mood swings, and hypothetically a whole bunch of other stuff. If the initial problem is bad enough that’s fine because those side effects aren’t a big deal compared to a full-blown body wide allergic reaction.
On the other hand, if it really is just a little mosquito bite and a little bit itchy, you probably don’t want to feel like you’re going through menopause just so you don’t scratch your leg. So, you get a topical formulation. You rub a little bit of steroid right on the bite and the itch goes away without all those pesky side effects from dosing your whole body with steroid.
This idea is also true for some other routes of medication administration as well. eye drops, inhalers and breathing treatments, for example, do the same thing. they put the medicine right where it is needed without giving the rest of your body a full blast dose of whatever it is.
I will note however that this is NOT perfect, and as another commenter said, you can get some bloodstream (systemic) absorption if you use a lot of whatever it is. This is easiest to see with inhalers, because the medicine has a lot of opportunity to get to the blood stream since the lungs have so many access points (although the principle is exactly the same for topicals). If someone takes too much albuterol (an inhaler medication that relaxes the muscles in the lungs to make it easier to breathe) their heart rate will get very high (up to the 150s or higher!) because that medication also increases the heart rate if it manages to get to the heart. This usually isn’t a big deal if you only use it once in a while as directed, because it doesn’t really get to the heart. BUT if you start puffing on it constantly because you have an asthma flair up it suddenly becomes very important.
The second reason I mentioned is controlled absorption, this is relevant for things like fentanyl or nicotine patches. In these cases, you want to give a set, continuous dose over a long period of time. These medicines have to be especially designed to absorb this way, because they don’t really do so naturally, the skin is a good barrier and doesn’t just let a whole bunch of stuff in. If you walk up and just touch some nicotine or fentanyl with your hand it really isn’t going to do anything. you could probably just brush it off on your shirt, and a quick hand rinse will 100% get rid of it and you won’t even notice it was there. But sometimes we WANT people to be getting the medicine all the time. For example, to help someone with chronic pain or someone who wants to quit smoking. A small, continuous dose does much better than giving it every once in a while, because it keeps the level of the medicine in the blood the same for longer. In these instances, a patch with medicine especially designed to go through the skin allows us to do that, and with some research we can fine tune the dose to exactly how much we want.
I’ve just written all this and realized I didn’t explain the exact mechanism of how the medicine gets through the bloodstream. I’m not actually an expert on this, but basically your skin is not a perfect barrier. It is very good, yes, but ultimately some things will slip through with enough prolonged contact, especially if designed to do so. The biological/chemical mechanisms for how this exactly happens are beyond my understanding unfortunately, but I’d be interested to read some responses from any biologists out there.
TL;DR: Your skin isn’t a perfect wall, but it is really good, and most of the time the reason for using topical medicine is to take advantage of the fact stuff doesn’t get through the skin very well.
Topical drugs are usually intended for local effects, meaning where you put the drug is where the drug should act. Take for example EMLA (eutectic mixture of local anesthetics) where you apply it on a part of your skin, that’s where the anesthesia will work. Topical drugs intended for systemic use are our patches (nicotine, nitroglycerin, scopolamine)
In cases of drugs going into to your blood stream, there is are specific physicochemical characteristics of the drug that will allow it to enter the skin. The two most important ones are molecular weight and lipophilicity (how much your substance can be attracted to oil). If you have a small mw and high lipophilicity, that drug can be absorbed through the skin, example is pesticides such as organophosphates or for more malicious examples nerve gas such as vx and sarin. If the drug does not have those properties it won’t be absorbed.
To answer the second question: no, since apple juice is not oil soluble, and won’t pass through your skin since your skin is relatively waterproof due to the horny layer
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