An active ingredient, such as particles of a pharmaceutical agent, is uniformly distributed within a transdermal vehicle, such as a gel or other viscous material. The method comprises filling said first container with a non-uniform mixture of a transdermal vehicle and said active ingredient, mounting the first container on a support member, and securing a second container to the first container. The first container has a restricted opening at its proximal end and a piston mounted movably within the first container for movement between a first position proximate said first container distal end to a position proximate said first container proximal end. The second container has a restricted opening at its proximal end and a piston mounted movably within the first container for movement from a first position proximate the second container distal end to a position proximate said second container proximal end. The first container restricted opening and said second container restricted openings are in open communication. The first container's piston is moved from its position proximate the distal end to a position proximate the proximal end and thereby forcing said mixture from the first container into said second container. The positions of the first and second container relative to said support member, are reversed and the process is repeated moving the mixture back and forth between the two containers until a uniform mixture is achieved.