clen on cycle is recomended since clen alone will indeiscriminantly burn muscle and fat since it raises your body temp by a degree or two.. so running a little bit of test while doing it will counter act the catabolic side effects.. it will not prevent you from gaining muscle, but it will you will gain less punds since you are burning weight the same time you are building it.. as far as i know it DOES NOT make you more efficient at breaking down food