Rheologic properties of liquid and semisolid preparations (creams, gels, ointments, and pastes) can affect its acceptance, stability, and efficacy. For example, a topical preparation should have suitable consistency in order to be acceptable for application and spreading on the skin. If the preparation has low viscosity (thin consistency) then it could spill or run off the skin during application. On the other hand, a high viscosity preparation ( thick consistency) could make it difficult to spread properly. It is also important to realize the effect of skin temperature on the viscosity of the applied product. Viscosity of multi-phase preparations (creams, pastes, emulsions, and suspensions) also affects their physical stability. Increasing viscosity makes the product more stable.
It is also expected that viscous products have increased contact time with the area of application. This is important for ophthalmic preparations. However, increased viscosity could make blinking difficult. Therefore, such preparations should be shear thinning and thixotropic. Thus, the blinking action creates enough shearing stress to reduce the viscosity of the applied formula (shear thinning). While at rest the formula quickly regains its original viscosity (thixotropy).
Rate of release of drugs from injectable depots is affected by its viscosity. A slow release formulation can have a thick consistency at the site of injection. However, passing a viscous formula through a needle is difficult and painful. Such preparations should be shear thinning and thixotropic. As force is being applied on the product in the syringe, the viscosity drops and the product passes with ease. However, when it rests at site, the original viscosity is regained.
Other areas of importance include manufacturing considerations such as proper choice of operating procedures, and equipment (e.g. mixing and packaging equipment). Finally, rheologic properties of a product affect the proper choice of its package.
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