Precipitated silica (silica dioxide prepared by precipitation/gas-phase method) acts as a highly effective adsorbent and flowability modifier in pharmaceutical excipients. Its core value stems from its physicochemical characteristics of high specific surface area and surface silanol groups (–SiOH), and it is widely used in tableting, anti-caking, and stabilization processes of oral solid dosage forms.

Pharmacological Mechanism of Adsorption
Precipitated silica binds to drug molecules, water, and impurities through a physical adsorption mechanism. Its specific surface area can reach 200–500 m²/g, and its porous structure provides a large number of adsorption sites.
In the granulation of traditional Chinese medicine extracts, it can adsorb excess water and viscous components, reducing particle stickiness, preventing agglomeration during granulation, and improving drying efficiency.
In wet granulation, as an anti-caking agent, it adsorbs moisture on the surface of fine powders, preventing particles from absorbing moisture and softening during storage, thus extending the stability of the preparation.
For easily oxidized or photosensitive active ingredients (such as vitamins and antibiotics), precipitated silica can form a physical barrier, slowing down degradation and improving the shelf life of the drug. Scientific Mechanisms of Flowability Modification
Modification Mechanism | Principle of Action | Practical Effects
Reducing Internal Friction | Surface silanol groups form a "lubricating layer" with drug particles, reducing van der Waals forces between particles | More uniform filling during tableting, tablet weight variation ≤ ±2%
Disrupting Agglomerate Structure | Micron-sized particles (1–10 μm) insert into the gaps between larger particles, breaking the "bridging effect" | Bulk density increases by 15–30%, flowability index (Hausner ratio) <1.25
Improving Compressibility | Porous structure undergoes elastic deformation under pressure, enhancing inter-particle bonding force | Increased tablet hardness, disintegration time shortened by 20–40%
Surface Hydrophobic Modification | Treatment with silane coupling agents (such as KH-560) reduces hydrophilicity and enhances compatibility with organic drugs | Prevents moisture absorption and caking, suitable for production in high-humidity environments
Precipitated silica is the mainstream choice for oral tablets due to its low cost and moderate purity (≥93%); fumed silica (purity >99%, particle size <10 nm) is used for injections and high-end sustained-release formulations, which have stricter requirements for sterility and particle size distribution.