Most of us probably don’t think about how much magnesium we’re consuming, but our bodies couldn’t function without it. We each typically contain about 25 grams of magnesium, more than half of which is stockpiled in bones. The rest is distributed in cells throughout the body, taking part in hundreds of essential chemical reactions, including building DNA and proteins and sending signals within our nervous systems and between our brains and the rest of our bodies.

But just because our cells—and especially our neurons—need magnesium doesn’t mean that extra magnesium will make them function any better. Not only that, but ingesting more of the element doesn’t mean it’s reaching your brain or really any specific part of your body.

In supplement pills, magnesium isn’t just on its own but part of a compound usually called a “magnesium salt.” This must dissolve to release Mg²⁺, the ionic version that our bodies use. How much of this actually gets into your system—versus gets excreted—depends on a variety of factors including the dose, your stomach contents, and how much magnesium is already in your body. In other words, the higher the dose, the less magnesium gets absorbed.

That’s all if the supplement dissolves in the first place. Magnesium oxide, which is usually the basis of the cheapest pills offered, doesn’t break down all that easily. In reality, magnesium oxide supplements mostly act as laxatives, drawing water into the intestines to flush out the excess salt.

This is where the marketing comes in. There are other types of magnesium compounds that do dissolve better. Magnesium citrate, for example, has been shown to have the highest bioavailability in laboratory tests. Despite the fact that all these salts still break down in the intestine, some supplement labels also claim that their specific chemical blend targets the magnesium directly to certain parts of your body, like your muscles or your brain.

These claims are often based on experiments using rodents, and the animals are typically supplied with magnesium long-term at higher doses than found in a pill. One study in rats found that a salt called magnesium L-threonate increased the amount of magnesium in the brain and improved some manifestations of learning and memory. This is the basis of some of the more overzealous marketing in the supplement world, which proclaims that this special salt will take the magnesium right to your brain.

There are some human studies, but that’s not really what they say. A few small randomized trials reporting improvements in some cognitive tests or sleep have been conducted—but there are several important caveats. Some studies tested a mixture of substances, lasted for only a few weeks, or analyzed a very limited number of outcomes. Many of these studies can also not be considered independent, because they were funded by supplement manufacturers or led by their staff.

Moreover, the studies—for extremely understandable reasons—didn’t measure magnesium content in the brain. So maybe a little more sneaks into your gray matter, but based on the scientific understanding of how these salts are processed, in all likelihood, a brain-focused magnesium supplement is still just an expensive laxative.

This story originally appeared on WIRED Czech Republic and Slovakia and has been translated from Czech.