Preservatives should be added at what stage when preparing a suspension in a glass mortar?

• A. Last
• B. During
• C. First
• D. After

Answer: (C)

Preservatives must be evenly distributed throughout the suspension to be effective. Adding the preservative to the vehicle at the beginning of the preparation ensures it dissolves or disperses completely before the dispersed drug particles are added, so every portion of the suspension contains preservative. If the preservative is added later, it may not mix uniformly, leaving pockets with insufficient preservative and allowing microbial growth. Early addition also helps maintain the proper chemical environment for preservative activity and reduces the risk of incompatibilities with components added later. Therefore, the preservative should be added at the start of the preparation.