lubricant

A lubricant is the tiny bit of slippery powder that stops a tablet from sticking to the machine. When powder is pressed in the die, the formed tablet must then be pushed out and the punch faces must release cleanly; without a lubricant the blend grips the metal, tablets stick or cap, and friction builds up. The lubricant coats the particles so everything slides smoothly.

The most common lubricant is magnesium stearate, a waxy, water-repelling powder used at well under one percent. Others include stearic acid, sodium stearyl fumarate and, for some uses, talc. The lubricant is added at the very end of blending and mixed only briefly, because over-mixing coats the particles too thoroughly.

That coating is also its drawback. Because magnesium stearate is hydrophobic, too much or too long a mix can waterproof the tablet, slowing wetting, disintegration and drug release, and it can also weaken the bond between particles and reduce tablet hardness. Formulators therefore keep the level and mixing time as low as the press will allow.