Which of the following best defines thixotropy in semi-solid dosage forms?

• A. Time-dependent decrease in viscosity under shear and recovery when at rest.
• B. Temperature-dependent viscosity changes with temperature.
• C. Instant decrease in viscosity with shear.
• D. Viscosity remains constant under shear.

Answer: (A)

Thixotropy is a time-dependent rheological behavior where a semi-solid's internal structure breaks down under shear and then rebuilds when the shear is removed. This causes the viscosity to drop gradually as the material is sheared, so it becomes easier to spread, and then recover its viscosity over time at rest, helping it stay in place after application. This reversible, history-dependent change distinguishes it from simply becoming less viscous immediately under shear (which is pure shear-thinning without time dependence) or from viscosity changes driven by temperature, which are not tied to shear history. It also differs from a Newtonian fluid that maintains constant viscosity regardless of shear.