If you read natural product labels, you might have seen a section called "other ingredients" and wondered what they do. Those "other ingredients" are excipients - things that don't make the formula better, but do make it possible. No one wants a product that is loaded with binders, fillers, flow agents, stabilizers, or other excipients - but without them, most capsules and tablets wouldn't be possible. Which excipients are needed, and in what amounts, will depend on the active ingredients in the formula, and on whether it is going in a capsule or a tablet.
Herbal tablets are made by compressing a powder inside a tablet die press with enough force (usually measured in tons) to fuse the powder into a solid form. Some ingredient powders will fuse together naturally, but most will not. These will require the addition of binders whose job is to fuse all the ingredients together, and may be up to half of the total formula by weight. The mixture may also need lubricants to keep the tablet from sticking to the die, and flow agents to help a sticky powder flow smoothly into the tablet die. Disintegrants may be added to help break the tablet apart in the stomach. Finally, many tablets are coated with a sugar or varnish coating either to seal oxygen and moisture out or to seal flavors or ingredients in.
Herbs in two-piece locking capsules don't need binders, and they aren't compressed as densely or as hard as tablets. But herbal materials vary in density, and some formulas may need a little filler more or less in order to measure correctly, into the capsule. Especially sticky powders may still need flow agents and/or excipients are typically less than 25% of the total capsule weight, and may be well below 1%. With capsules, the capsule itself is usually the biggest excipient by weight.
There are two broad types of capsules in common use - gelatin and HPMC. Gelatin has been the gold standard of capsules for many years, but gelatin is refined from animal byproducts, so it isn't vegan-friendly. Common sources for gelatin include beef (not acceptable by the Hindus), pork (not Kosher or Halal), and chicken, which is more acceptable to consumers than the other kinds, but still not vegetarian or vegan-friendly.
Vegetarian or vegan capsules are made of hydroxypropyl methylcellulose (HPMC), a clear form of cellulose usually refined from wood pulp. Cellulose is a dietary "fiber" and is the rigid part of the cell walls of a plant. HPMC solves the vegetarian issue posed by gelatin, but the long chemical name sometimes sounds scary to consumers. In the past, HPMC capsules could be more brittle than gelatin, and more likely to crack and break during encapsulation. They have improved a lot recently.
Other excipients in capsules may include:
Cellulose powder - a filler typically made from refined pine or cotton fibers. Pure cellulose is inert, inexpensive, blends well, and is very similar to the herbs it is used with as a filler. But it is not of much value as a flow agent.
Microcrystalline cellulose (MCC) - a more refined form of cellulose made from wood pulp, with a smaller and more uniform granula size. MCC is useful as a flow agent, and as a lubricant, but is much less effective in those roles than other materials. Some consumers assume that it is synthetic, usually just based on the sound of the name, but it is a natural and vegan-friendly product.
NuRice - a proprietary and non-GMO form of rice flour, which is gluten-free and of consistently high quality. It has good lubricant properties.
Rice flour - a common filler, used because it is inexpensive, gluten-free, and generally accepted by consumers. Pure rice (even so-called "glutinous" rice) is gluten free, but some rice is GMO.
Silica - a good flow agent that is almost completely inert, and not readily absorbed by the body. It is safe when taken orally, but should not be inhaled, as it can produce a lung condition called silicosis.
Stearic acid - a saturated fatty acid that is a very effective lubricant. It can be derived from either animal or plant sources, but animal sources are more common. Plant sources include cottonseed and palm oils, but cottonseed oil often carries high pesticide residues. One study associated high doses of stearic acid to disruption of T-cell activity, and others suggest it may slow absorption in the body. Most consumers get far more stearic acid from their diet than could ever be contained in capsules. Still, why use it when there are apparently better alternatives?
Magnesium stearate - stearic acid with a magnesium ion added. Also a very effective lubricant, but shares many of the same concerns as stearic acid.