Eli5: Why do companies put caffeine in soaps?

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Does your body absorb it somehow? Does it help as a moisturizer?

In: Biology
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Technically it can get absorbed through the skin, but probably not enough to have any real effect on you in the time it’s going to sit on your skin in a shower.

Soap makers add it for marketing.

There have been a few studies, according to a “board certified dermatologist at Modern Dermatology in seattle” – “Topical caffeine is best known for its antioxidant properties. With topical antioxidants, you can decrease the harmful effects of free radicals created by environmental pollutants and UV radiation, which are what cause fine lines and wrinkles.” He goes on: “Caffeine has been shown to constrict blood vessels when applied to the skin. It is thought that caffeine may temporarily constrict these tiny blood vessels, somewhat masking the darkness under the eyes. Furthermore, caffeine-enhanced skincare products can decrease inflammation and irritation. They may reduce swelling and feel ‘soothing.’ ”

Caffeine is a natural insecticide. Plants produce it to kill insects that eat the bit of the plant that produces the caffeine. It basically kills the bug with a caffeine overdose.

Scented soaps can attract insects, because they smell like food. This is a bigger problem with animal based soaps, since animal fats *are* food.

The caffeine in the soap kills bugs that try to eat it.

Topical caffeine can’t really get into your bloodstream to the levels necessary to have the same effect as say… a caffeinated drink. But it can penetrate into the upper levels of your skin and cause minor vasoconstriction (reduced redness / dark circles) and a mild tightening effect, and so it’s popular in eye creams. But even if these claims are true, the effect would be purely temporary and short-lasting. Any claims about it improving your skin long-term are not currently supported by the evidence.

It’s more likely that including it in other products like shampoo is more akin to the “cold” sensation of minty toothpaste: i.e. it has no effect whatsoever on actual oral hygiene, but people have come to associate the cold feeling with how toothpaste “should” feel, and will reject toothpastes that don’t add it.