Impurities in pharmaceutical substances are unwanted or unintended materials that contaminate the drug product or active pharmaceutical ingredient (API). These impurities may originate from various sources such as raw materials, reagents, solvents, manufacturing equipment, processing procedures, degradation products, or microbial organisms.

The impurities may differ in nature, quantity, and toxicity. They may be organic or inorganic compounds, elemental substances, particles, or microorganisms. They may also have different physical, chemical, and biological properties, such as color, taste, odor, solubility, stability, or toxicity.

Some impurities may pose a health hazard to patients or alter the therapeutic efficacy of the drug product. For example, genotoxic impurities may cause mutations or cancer, while residual solvents may cause toxicity or affect pharmacokinetics. Other impurities may affect the product quality or stability, such as moisture, oxidants, or metal ions.

The regulation of impurities in pharmaceutical substances is governed by several international, national, and regional guidelines and standards. These include the International Council for Harmonisation (ICH) guidelines, the Pharmacopoeias, and the Good Manufacturing Practice (GMP) regulations.

The detection, identification, and quantification of impurities in pharmaceutical substances require various analytical techniques, such as chromatography, spectroscopy, mass spectrometry, or microbiological testing. The risk assessment and control of impurities involve various strategies, such as setting limits, conducting impurity profiling, using appropriate raw materials, optimizing manufacturing processes, or implementing quality control measures.