Breaking the straight line
Most pharmaceutical liquids and semisolids are not Newtonian. Their viscosity changes depending on how fast you shear them, so their rheogram is a curve, not a straight line. We call all of this non-Newtonian flow. Far from being a nuisance, this is exactly the behaviour a formulator designs in on purpose — it is what lets a product be thick at rest and thin while you use it.
When a material is non-Newtonian, a single number cannot describe it. Instead we talk about its apparent viscosity — the viscosity it happens to show at a particular shear rate. Quote it without a shear rate and, like quoting viscosity without temperature, the figure is incomplete.
Shear-thinning: thinner as you push harder
By far the most common and most useful behaviour is pseudoplastic flow, also called shear-thinning: the faster you shear, the lower the apparent viscosity. At rest the material is thick; under stress it flows freely. Most gels, many lotions, and polymer-thickened liquids behave this way. Inside, long polymer chains are tangled at rest but line up with the flow when sheared, so they slip past one another more easily.
Shear-thickening: thicker as you push harder
The opposite, rarer behaviour is dilatant flow, or shear-thickening: the faster you shear, the higher the apparent viscosity. Stir it slowly and it pours; stir it fast and it seizes up. The classic kitchen demonstration is a thick paste of cornstarch in water. It typically appears in highly concentrated suspensions of small particles: at rest the liquid just fills the gaps between particles, but rapid shear forces the particles apart, the packing loosens, and there is no longer enough liquid to lubricate them — so resistance shoots up.
Dilatancy is usually a problem rather than a feature. If a concentrated suspension is dilatant, pumping it too fast during manufacture can make it set solid in the pipe or jam a mixer. The cure is to lower the solids content, change the particle size, or simply process it more gently at a lower shear rate.