Moisture is one of the most prevalent risks to pharmaceutical stability. From oral solid dosage (OSD) forms such as tablets and capsules to inhalation devices, fluctuations in humidity can compromise drug quality, alter dissolution profiles, and shorten shelf life. Excess moisture can catalyze hydrolytic degradation or polymorphic changes, while overly dry conditions may cause excipients and gelatin capsules to lose mechanical integrity.
Traditionally, packaging has relied on desiccants such as silica gel and molecular sieve to mitigate moisture ingress. These technologies remain widely used and effective in many applications. However, their inherent adsorption behaviours – either too gradual or too aggressive – do not always align with the specific needs of modern drug products. This has prompted interest in engineered equilibrium systems designed to maintain a target relative humidity (RH) rather than simply reducing it as far as possible.
This article reviews the strengths and limitations of silica gel and molecular sieve, and explores the role of equilibrium humidity stabilisers, such as EQIUS®, in balancing product stability with packaging efficiency.
Silica Gel:
Versatile and Economical Moisture Control
Silica gel, composed of amorphous silicon dioxide, has long been favoured in pharmaceutical packaging. Its internal network of pores adsorbs water vapor through capillary condensation, allowing it to function across a broad RH range.
Advantages of Silica Gel
• Cost-Effective and Scalable: widely available, suitable for high-volume packaging with options to further moderate costs through bulk purchasing or optimised usage.
• Versatile formats: available as packets, canisters, capsules, or washers, adaptable to different packaging designs.
Limitations of Silica Gel
• Limited efficiency at low RH: struggles to achieve or maintain ultra-low humidity levels required by highly hygroscopic APIs or the amount of silicagel required to achieve low RH is very important.
• Potential overdrying: in certain cases, may reduce RH below optimal thresholds, causing gelatin capsule brittleness or tablet friability and can stress sensitive dosage form.
Silica gel remains a practical solution for many products where moderate moisture control suffices, but may fall short when precise or low-RH environments are essential.
Molecular Sieve:
Targeting Low RH with High Capacity
Molecular sieve, typically a crystalline aluminosilicate (zeolite), offers a sharper tool for moisture control. Its uniform pore sizes selectively adsorb water molecules, enabling aggressive uptake even at very low ambient humidity.
Limitations of Molecular Sieve
• Risk of overdrying: extreme adsorption can reduce humidity below the stability window for certain formulations, leading to capsule brittleness or altered drug release profiles.
Molecular sieve is therefore particularly useful for highly moisture-sensitive products, diagnostics, or biologics, but requires careful evaluation to avoid overdrying effects.
