related substances

Related substances are the impurities that are chemically related to the drug itself — close cousins of the active molecule, most often its degradation products or synthesis by-products. The name signals they are not random contaminants but compounds that arise from the drug's own structure and chemistry.

They are typically controlled by a chromatographic test that separates and quantifies each impurity, with the specification setting a limit for any individual unspecified impurity, named limits for specific known ones, and a limit for total impurities. As a product ages, degradation products such as hydrolysis or oxidation products tend to grow, so related substances limits are a key part of demonstrating stability and setting shelf life.

Related substances is a subset of the broader category of impurities, focused on the organic, drug-derived ones; it usually excludes residual solvents, inorganic and elemental impurities, which are controlled by separate tests. Tightening these limits often drives formulation choices like antioxidants, protective packaging, or refrigerated storage.