weight variation

Weight variation simply checks that individual units do not differ too much in weight from one another or from the average. It is the fast scale-based cousin of content uniformity: instead of chemically assaying each unit, you weigh them, relying on the assumption that for a well-mixed, high-dose product, weight tracks drug content.

A number of units (such as twenty tablets) are weighed individually; the mean is calculated and each unit compared against percentage limits around that mean. Pharmacopoeias allow only a small number of units to fall outside a tighter band and none outside a wider one. Where it is permitted, the result can also be converted to an acceptance value to satisfy uniformity of dosage units.

Weight variation is allowed only when content can reasonably be inferred from weight — typically high-dose units where the active makes up most of the unit, solutions filled into single-dose containers, or freeze-dried powders. When the active is a small fraction of a low-dose unit, weighing cannot detect blend non-uniformity, so content uniformity is required instead.