impurity

An impurity is anything in a drug product that is not the active ingredient or an intended excipient — an unwanted extra. Even a very pure medicine carries trace amounts of other substances, and the job of the specification is to keep each one below a level shown to be safe.

Impurities are grouped by origin. Organic impurities include unreacted starting materials, by-products and intermediates from synthesis, and degradation products formed as the drug ages. Inorganic impurities cover residual catalysts, reagents, and elemental impurities such as heavy metals. Residual solvents are the volatile liquids left from manufacturing. Regulatory guidelines set reporting, identification, and qualification thresholds that rise or fall with the daily dose.

Not every impurity matters equally: limits are tied to toxicity, so a tiny amount of a genotoxic impurity may be tightly capped while a more benign one is allowed at higher levels. Controlling impurities is central to both safety and stability, since many appear or grow over a product's shelf life.