Magnesium stearate: what the filler in a tablet is for

Turn a tablet pack over and the ingredient list usually falls into two halves. The first is what you bought. The second is a short run of names that read like a chemistry lesson, and it is the half most people either skip or look up online, where they find that one of those names has a reputation.

Magnesium stearate is that name. It is the standard lubricant of tablet manufacturing, and it has become the ingredient that packs advertise leaving out. What follows is what it does in a tablet, why something like it has to be there at all, and which question about fillers is worth asking - because it is not the one printed on the front of the packs that make a feature of this.

What is an excipient in a supplement?

The word for that material is excipient. An excipient is any substance in a tablet or capsule present for a reason other than being the ingredient the product is built around: to add bulk, to hold the tablet together, to help powder flow through machinery, to stop it sticking, or to keep it stable on a shelf. The term comes from medicines manufacturing. In food supplements the same substances do the same jobs under food law rather than medicines law, which changes how they appear on the label and not what they do in the press.

Why 100 mg of powder cannot be a tablet on its own

The need for them is physical, and it is easier to see if you picture the machine. A tablet press fills a small steel cavity, called the die, with a measured volume of powder, squeezes that powder between two steel punches at a force measured in kilonewtons, and pushes the result out. Every stage has requirements. The powder has to fall into the die at the same density thousands of times an hour, or the tablets come out at different weights. It has to compact into something that survives a blister foil, a courier and a coat pocket. It has to leave the punch faces cleanly, and come apart again once swallowed.

A hundred milligrams of milled plant will do none of that on its own. It is roughly the weight of two grains of rice, and pressed alone it would give you a tablet too small to handle and too fragile to blister-pack. So something is added for volume, and the usual choice is microcrystalline cellulose: purified plant fibre, milled to a controlled particle size. It deforms rather than shatters under pressure, so it fills the tablet out and binds it at once, and formulation literature calls it the most widely used direct-compression excipient there is. In food law it is E 460(i), and EFSA re-evaluated the celluloses in 2018, concluding that no numerical acceptable daily intake was needed and that there was no safety concern at the reported uses and use levels.

Spotting these on a label is straightforward once you know the format. Ingredients are listed in order of weight, heaviest first, and any food additive must be declared by the name of its functional class followed by either its specific name or its E number. So bulking agent: microcrystalline cellulose is not a brand being coy. That wording is required. Where a pressing aid is a food in its own right - rice flour, say - it appears simply as itself.

Why is magnesium stearate used in supplements?

Powder pressed hard against polished steel sticks to it. In tableting the failure has a name - punch sticking - and it means material adhering to the punch tip instead of leaving with the tablet. Once a punch face has picked up a film, every tablet after it comes out marked, and the line stops to be cleaned. Friction turns up again on the way out, because the tablet has to be ejected up the wall of the die.

What a tablet lubricant actually does

A lubricant is what stops that. Mixed into the blend in small quantities, magnesium stearate coats particle surfaces and reduces friction between particles and along the die wall, which lowers the compaction force, the ejection force and the heat generated in the process. Work published in 2023 on how it does this found that its effect on sticking tracks the proportion of the tablet surface the lubricant actually covers, rather than the amount that went into the blend - which is why blending time matters as much as quantity.

The quantities are small by design. Published tableting work puts the working range at nought to about two per cent of blend weight, and formulators sit near the bottom of it, because lubricant is not free of consequences: it reduces the tensile strength of the tablet, and being water-repellent it can slow the tablet's break-up. Too little and the press jams. Too much and the tablet is soft and slow. The amount used is the smallest that keeps the line running, which on a small tablet is a few milligrams.

As for its standing in law, magnesium stearate is a magnesium salt of stearic acid, one of the fatty acids covered by the food additive E 470b, magnesium salts of fatty acids, which sits on the list of additives approved for use in Great Britain. EFSA re-evaluated E 470a and E 470b in 2018 and concluded that there was no need for a numerical acceptable daily intake, and that the additives were of no safety concern at the reported uses and use levels.

That is a regulator's finding about a food additive at food levels, and it is the only sort of statement we are going to make about it. What magnesium stearate does or does not do inside a person is not something a supplement brand is in a position to tell you, and that goes for the brands assuring you it is nothing to worry about as much as the ones implying it is. Both are selling. A brand that answers a question about its own ingredients with a reassurance has given you nothing you can check.

How much of a supplement tablet is filler?

Presence is not the complaint that survives contact with a tablet press. Proportion is. A tablet that is nine-tenths bulking agent and one-tenth of the material named on the front is a different object from one built the other way round, and the difference is not a matter of opinion. It is arithmetic, and the label gives you enough to do it.

How to work out how much of a tablet is filler

Start with the weight of one tablet. Most packs do not print it directly, but nearly all print the net quantity of the pack, because food labelling requires it. Divide that by the number of tablets and you have the average tablet weight. Then add up the declared amounts of the things you actually bought - the milligrams printed beside each vitamin, mineral or plant. The gap between the two figures is everything else: bulking agent, binder, lubricant, any coating.

Then read the order of the list, which does the same job faster. If the first name is a cellulose, the cellulose outweighs everything else in the tablet, including whatever is on the front of the box. That is not automatically damning. A low-dose vitamin tablet has no other way to exist, and nobody is cheated by a tablet built round 10 mg of zinc. It matters when a brand has charged a botanical price for a botanical-shaped tablet that is mostly not botanical.

This is also the point at which the arithmetic can be taken away from you. If the plants are grouped into a proprietary blend on the back, declared as a single total with no individual weights, the sum cannot be done at all. You know the size of the parcel and nothing about what is inside it. A pack can carry a magnesium stearate free flash on the front and a blend on the back, and the flash is the part the shopper will remember.

Ours is a six-plant tablet holding 100 mg of plant material: Ginseng 25 mg, Maca 25 mg, Tribulus 20 mg, Guarana 10 mg, Ashwagandha 10 mg, Catuaba 10 mg, at a maximum of one tablet in any 24-hour period. Every one of those milligrams is printed, so the subtraction above can be run on our pack by anyone holding it. The tablet weighs more than 100 mg, because 100 mg of milled plant is not a tablet, and the difference is pressing material, declared in the same ranked list in the format the rules require. Read that line on the carton rather than here: an article ages and a carton does not. For the laboratory results behind a particular run, quote the batch code from the pack to hello@maleup.co.uk.

Does 'magnesium stearate free' mean anything?

It is a good piece of marketing, and worth taking apart for that reason. It is a negative claim, so it costs nothing to make and is awkward to argue with. It implies a problem without ever stating one, which keeps it clear of the rules governing what a food label may say about health. And it is true. It is simply true about something that was never the interesting part of the tablet.

What brands use instead of magnesium stearate

The engineering behind it is unremarkable. A manufacturer who wants the claim uses a different lubricant, such as stearic acid or a hydrogenated vegetable oil; or applies lubricant to the tooling instead of mixing it into the blend; or fills capsules rather than pressing tablets; or runs the press slower and absorbs the cost. These are ordinary production decisions. None of them changes the species in the powder, the number of milligrams of it, or whether anyone tested the batch.

So the phrase is best read as a signal about what a brand believes its customers respond to. Sometimes it sits beside genuinely full disclosure. Often it is doing the work the ingredient weights ought to be doing, and the giveaway is what is absent from the back: no botanical species in Latin, no per-plant milligrams, no batch code, no certificate.

What to look at instead is duller and more useful. Look at the weight of the material you are buying, per tablet, printed as a number rather than folded into a total. Look for the species or the specific chemical form beside it, so you know what the number refers to. Do the tablet-weight subtraction if the ratio matters. Then find the batch code and ask whoever sold it to you for the certificate of analysis behind that batch, because that is where identity and contaminant testing live. A brand that can produce all four has told you more than any front-of-pack claim about an absence.

None of this makes fillers virtuous. They are the price of the format. A tablet is a manufactured object, and manufactured objects need material that does the manufacturing. The honest version of the argument is not this pack contains an excipient. It is this pack contains more excipient than product, and will not tell you the ratio. The first is a fact about tablets. The second is a fact about a brand, and it is printed on the back, in weight order, for anyone who turns the box over.

Common questions

† Describes the traditional use of these botanicals (UK 'on-hold' botanical claims under retained Regulation (EC) 1924/2006). Not authorised EFSA health claims. AllMaleUp is a food supplement, not a medicine.