Briefly; Impurities are the main huge tree! while related substances are considered branch of that tree.

A good guidance is ICH Q3A.

 In details; according to ICH Q3A; Impurities can be classified into the following categories:

 Organic impurities (process- and drug-related; related substances)

 Inorganic impurities

 Residual solvents

Organic impurities can arise during the manufacturing process and/or storage of the drug substance. They can be identified or unidentified, volatile or non-volatile, and include:

- Starting materials

- By-products

- Intermediates

- Degradation products

- Reagents, ligands and catalysts

Inorganic impurities can result from the manufacturing process. They are normally known and identified and include:

 Reagents, ligands and catalysts

 Heavy metals or other residual metals

 Inorganic salts

 Other materials (e.g., filter aids, charcoal)

Solvents are inorganic or organic liquids used as vehicles for the preparation of solutions or suspensions in the synthesis of a new drug substance.