The Pharmaceutics and Compounding Laboratory
Pharmaceutical Solutions III:
Ophthalmic Solutions

Viscosity

Viscosity measures the resistance of a solution to flow when a stress is applied. The viscosity of a solution is given in poise units. The unit centipoise (cp or the plural cps) is equal to 0.01 poise and is most often used in pharmaceutical applications. Compounds used to enhance viscosity are available in various grades such as 15 cps, 100 cps, etc. The grade number refers to the viscosity that results when a fixed percentage aqueous solution is made. Generally the solutions are 1% or 2% and the viscosity is measured at 20oC.

Viscosity enhancers are used in ophthalmic solutions to increase their viscosity. This enables the formulation to remain in the eye longer and gives more time for the drug to exert its therapeutic activity or undergo absorption. Commonly used viscosity enhancers and their maximum concentrations are given in the table below.

Viscosity Enhancer Maximum Concentration (%)
Hydroxyethylcellulose 0.8
Hydroxypropylmethylcellulose 1.0
Methylcellulose 2.0
Polyvinyl alcohol 1.4
Polyvinylpyrrolidone 1.7

The most common viscosity desired in an ophthalmic solution is between 25 and 50 cps. The actual concentration of the enhancer required to produce that viscosity will depend on the grade of the enhancer. For example, if methylcelluse 25 cps is used, a 1% solution will create a viscosity of 25 cps. If methylcellulose 4000 cps is used, a 0.25% solution provides the desired viscosity. Standard references give tables of viscosities produced by percentage solutions and grades of ingredients.