Caffeinated soap has been making the rounds for a few years now, and the basic logic is that you wash first thing in the morning, and drink coffee in the morning, so of course you want caffeine in your soap.

Unfortunately, reality doesn’t hold up to this idea. In order to push a chemical (such as caffeine) across a membrane (such as your skin) you have to cover as much of the surface as possible and let it stay on the surface for as long as possible. And in the case of caffeinated soap, according to studies, if you were to soap your entire body and let the lather sit on your skin for an hour, you would only absorb 40 milligrams of caffeine. Maximum. In an hour. A can of diet soda contains 45 milligrams, and a cup of coffee contains between 100-200 milligrams depending on how you brew it.

The caffeine you don’t absorb from the bar gets washed down the drain and ends up in the river, which isn’t really a big deal, except that you paid $9 a bar for it.