Magnesium stearate

Magnesium stearate appears in a very large share of capsules and tablets, and it generates more suspicion per milligram than almost anything else in a supplement. Most of that suspicion is misplaced. One part of it is not, and the two get tangled together constantly.

What it is doing there

It is a lubricant. Powders moving through high-speed tableting machinery stick to the equipment and to each other. Magnesium stearate coats the particles so they flow, which is why it appears in tiny quantities near the end of an ingredient list.

It is not an active ingredient, it is not a preservative, and it is not there to bulk out the product. It is there so the machine works.

The safety claims, and why they do not hold

The arguments made against it online tend to be some version of: it suppresses the immune system, or it forms a film that blocks absorption.

Both trace back to studies that do not say what they are quoted as saying. The immune claim comes from work on isolated T cells in a dish exposed to stearic acid directly, which is a very long way from a person swallowing a few milligrams inside a capsule. The absorption claim rests on dissolution behaviour observed under specific laboratory conditions and does not translate into meaningfully reduced uptake in a finished product.

Underneath both is the same error this site keeps returning to: treating a hazard shown at one dose by one route as though it described an ordinary exposure. Stearic acid is a fatty acid found in beef, cocoa butter and cheese, and you eat far more of it in a normal day than a supplement contributes. The reasoning is set out at length in the guide on hazard and exposure.

If you came here worried the lubricant in your vitamin was harming you, the evidence does not support that worry.

The question that is real

Where the stearic acid came from.

The US regulation defines magnesium stearate as the magnesium salt of stearic acid “obtained from edible sources”. Edible. Not vegetable, not animal. The regulation has no reason to care, because for its purposes the two produce the same substance.

In practice stearic acid is made from fats, and those can be palm or other vegetable oils, or they can be tallow. The finished magnesium stearate is chemically identical either way and the ingredient list says the same seven letters.

This is precisely the situation with Mono- and diglycerides, and it is not a coincidence: both regulations use the same phrase. Anyone who has already learned that E471 can come from lard has learned this one too.

How to find out anyway

Vegetable magnesium stearate on the label. Manufacturers selling to people who care about this say so, because it is a selling point. When the words are there they are reliable.

A vegan, halal or kosher certification mark, which covers the supply chain rather than the packet.

Asking. Supplement companies field this question constantly and usually answer it quickly.

Is it worth avoiding

If your reason is animal origin or religious observance, then yes, and the routes above are how you do it.

If your reason is that you read it was harmful, then no, and the capsules sold as “no magnesium stearate” are usually charging a premium for the absence of a lubricant rather than for anything you gain. That is a marketing position built on a worry, which is the pattern this whole site exists to interrupt.