Caffeine is a weird drug. On the one hand, it's a stimulant that frequently leaves people feeling jittery. On the other hand, some people find it gives them an uncanny amount of focus and energy. Studies have shown that it can help with attention, weight loss, and productivity.
As such, caffeine has both been glorified and stigmatized over the years. Some people hate it and some people recommend it for everything. The reality, as you might expect, falls somewhere in the middle.
And then we have coffee. Coffee, the blackened lifeblood of the working class around the world. The dark fuel that powers all industry, all technology, all development. Coffee is something millions of people can't live without, and it's so prevalent that people keep finding more and more uses for it.
Like coffee scrubs!
A coffee scrub is a way to re-use coffee grounds after brewing a pot of coffee for the morning. The grounds are a mild abrasive, which means when they're mixed with the right ingredients, it can make a great little exfoliating scrub for your skin. It's a mechanical scrub, meaning it exfoliates through abrasiveness and mild wiping and massaging, not through chemical reactions.
That doesn't mean coffee is inert, though. A coffee scrub has a lot of stuff going on, but the biggest among them is the caffeine. Remember, brewing coffee is a lot like brewing tea; the water you use leeches out much of the nutrients and caffeine from the grounds, but not all of it, not by a long shot.
A coffee scrub has caffeine in it, that's not up for debate. The question is, does that caffeine do anything?
Can Caffeine Absorb Through the Skin?
First, the mechanical question. Does caffeine absorb through the skin? If it does, great! Then we can start looking at what kind of effects it might have. If it doesn't, though, then there's no point in discussing this any further. If caffeine doesn't penetrate the skin, then it's just a needless buzzword with no value in a skincare product.
Well, there's some good news and some bad news here.
First, the good news. Caffeine does, in fact, absorb through the skin. Good, right?
Well, not so fast. Just because something can be absorbed through the skin doesn't mean it does it very well. In fact, your skin is very good at keeping things out. Studies indicate that in one square centimeter of skin, it takes about an hour to absorb around 2 micrograms of caffeine. 
What this means is that, on its own, a coffee scrub is not going to get much caffeine into your bloodstream. You would have to slather your whole body with it and leave it in place for several hours to get enough caffeine to equal one cup of relatively weak coffee. In that time, the topical application of coffee and other ingredients is more likely to irritate your skin than anything else.
Now, there are some other ingredients that are called carriers, which are very, very good at penetrating the skin. They also carry other ingredients with them. Some of these are very carefully used in some skincare products, but because they can be dangerous – bad things can penetrate just as much as good things – they aren't common.
For example, the chemical compound dimethyl sulfoxide is extremely good at penetrating the skin, so much so that within minutes of it touching your skin, it has penetrated and circulated through your system enough that you can taste it. This would be very good at getting coffee into your blood, but at the same time, it's not something you want to be using in an uncontrolled fashion. You're not going to be using this in a homemade scrub, not without a chemistry degree and a healthy disregard for personal safety.
The Purpose of Topical Caffeine
Now, we have a little bit more good news here. If you want caffeine in your bloodstream to perk you up, that's fine! There are thousands of different products available for exactly that purpose, from caffeine pills and black coffee to caffeine-infused snacks, soft drinks, and everything in between.
Caffeine is a vasoconstrictor. What this means is that, when caffeine reaches blood vessels, it makes them shrink a bit.
What this means, is that when applied topically in a scrub or skincare product, that caffeine content can reduce inflammation, reduce bloodflow to a given area, and reduce puffiness and redness.
This isn't necessarily going to treat inflammation; caffeine is an anti-inflammatory, but it's not necessarily a good one compared to many other sources. What it means is that it can help hide the effects of inflammation, but not treat the cause. On the skin, this usually means redness, puffiness, and sometimes those dark circles under your eyes.
Still, though, the amount of caffeine that makes it into your system is relatively minimal. Thus, the effects it can have on your skin are minimal as well. A little bit of eye puffiness caused by a late night out, sure, caffeine can hide that. A bit of a flush across your cheeks? Sure, use the coffee scrub and it should clear up for you.
Deep eye bags stemming from genetic predisposition? Deep rosacea that makes your cheeks turn bright red? Unfortunately, that topical application of caffeine isn't going to do a whole heck of a lot there.
Some people believe that a topical caffeine product can help with cellulite. Cellulite is caused by uneven texturing of the fat beneath the skin, and there's some evidence to suggest that caffeine can stimulate the enzymes that break down fat. Unfortunately, at the amount of caffeine you're getting topically, and the amount of fat that needs to break down to smooth out cellulite, it's not likely to be very effective.
The Benefits of a Coffee Scrub
All of the above in mind, a coffee scrub is not useless. The primary benefit of a coffee scrub is for the mechanical exfoliation. Rubbing off dead skin and grime, cleansing it away, and leaving your skin open to a moisturizer (with or without additional caffeine in it) is a benefit unto itself.
Coffee grounds are generally a mild enough abrasive that they don't really damage your skin when mixed up into a proper coffee scrub, but that does tend to vary based on the kind of coffee grounds you're using.
Another benefit to a coffee scrub is simply that it's getting another use of a product you might have otherwise just discarded. You're still going to discard the coffee grounds when you're done with them, but if you're using a coffee scrub rather than a product you buy just for the exfoliation properties, you're saving the world a little bit of industry and a little bit of plastic from whatever container that new scrub came in. You can also then take the used scrub and make it part of a compost pile, if you have the facilities to do that.
The Drawbacks of Coffee Scrubs
Perhaps one of the biggest possible drawbacks to a coffee scrub is rebound redness. Remember that caffeine is a vasoconstrictor, and vasoconstrictors wear off. In fact, it has been found that when a vasoconstrictor wears off, often times your blood vessels will overshoot in the other direction to compensate. In other words, those blood vessels open up even wider than they were before, until they settle back down to baseline.
What this means is that if you're using a caffeine-based skin product or a coffee scrub to try to treat redness in your skin, it will only work as long as you're using it. When you stop using it, your blood vessels will open up way further than before, and it will lead to even worse redness than before, until your skin settles back down.
Incidentally, this is the same mechanism that causes caffeine withdrawal headaches, only the blood vessels doing it are in your brain and throughout your body, not just in your skin. 
Another possible problem you can encounter is that caffeine can actually inhibit the formation of collagen. As we know, collagen is critical to the elasticity and youthful appearance of your skin. By inhibiting the production of more collagen, you accelerate the potential damage your skin takes over time, and how quickly it ages.
Now, again, this all going to be a minor effect, because of how little caffeine actually penetrates your skin. Still, it's something you should be aware can happen.
Some people have a bit of concern about "overdosing" on caffeine because of all of the skincare products and other sources of caffeine available in daily life. Honestly, though, this shouldn't be a concern.
For one thing, the amount of caffeine getting into your system through your skin is minimal. You would have to be using a combination of caffeine and a very effective carrier like DMSO to get more than a minimal amount in through your skin, and even then, you'd have to get a lot of that caffeine plus a lot more that you drink or consume before you reach dangerous levels.
The danger level of a substance is measured by its LD50, which is the "median lethal dose", or the amount of the substance necessary to kill roughly half of the people who consume that quantity. For caffeine, the LD50 is 150-200 milligrams per kilogram of body mass. For an adult who weighs 150 lbs, that means you would have to consume an equivalent of 100 cups of coffee in a day to have a 50% chance of dying from it.
Sure, caffeine can have a variety of negative effects long before it reaches a lethal level, but even that is a lot more than what you're getting from a few cups of coffee and a scrub. Don't worry about it.
Should You Use a Coffee Scrub?
So, is a coffee scrub good for you or not? Well, it depends on why you want to use it.
If you want to use a coffee scrub because you already use exfoliating products, you already brew your own coffee, and you want to reuse the coffee grounds instead of just throwing them away? Sure, go right ahead. There's no reason not to use a coffee scrub in this case. You may have to play around for a while to find a formula that works best for you, but it won't be too difficult.
If you want to use a coffee scrub to try to counteract a bit of minor redness and puffiness that consistently affects your skin? Sure, you can give it a try. You may want to try a variety of different caffeine-included skincare products, since it's hard to estimate a given dose, though. Most skincare products don't tell you how much caffeine is in them, and you will have a hard time judging how much is in a brewed set of coffee grounds. Just try different products, see what works, and be aware of rebound redness.
If you want a coffee scrub to take the place of your daily cup, you're probably not going to get that much out of it. You can try to layer on several different caffeine-containing products, like a coffee soup or body wash, a scrub, and even a shampoo, but a cup of coffee is still going to provide you more caffeine in an easier fashion.
Really though, it comes down to what works for you and what you enjoy. If you like using a coffee scrub, go ahead and do it. If you're just forcing yourself to try it because of the caffeine, well, you can higher concentrations of caffeine with a cup of coffee, so don't force it. Just do what works best for you!
