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.

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