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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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.
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