a Ficks first law the rate of diffusion or transport across a membrane (dc/dt is proportional to the difference in drug concentration on both sides of the membrane dc/dt=P(CrC2) in which Ci and C2 refer to the drug concentrations on each side of the membrane and p is a permeability coefficient or constant.◼ Fick’ s first law: the rate of diffusion or transport across a membrane (dc/dt) is proportional to the difference in drug concentration on both sides of the membrane. -dc/dt=P(C1 -C2 ) in which C1 and C2 refer to the drug concentrations on each side of the membrane and P is a permeability coefficient or constant