Square root of time dependence of matrix formulations with low drug content.

One of the conditions of derivation of the Higuchi square root law is that A/epsilon greater than S/2 where A is drug content per cubic centimeter of matrix tablet, epsilon is the porosity, and S is the solubility of the drug in the dissolution medium. In actuality, A/epsilon should be larger than S. It is shown in this work that a similar square root equation can be derived when A/epsilon less than S. Experimental data are presented supporting the equation Q = A(Dt)1/2, where Q is the amount of drug released per square centimeter at time t and D is the diffusion coefficient.