a slow - release pharmaceutical agent which comprises fumaric acid and / or dl - tryptophan in addition to one or more pharmaceutically active substances , and a process for preparation of the same are disclosed . among many organic acids and amino acids , fumaric acid and dl - tryptophan are remarkably useful for extending the duration of release of a pharmacologically active substance when fumaric acid , dl - tryptophan or both is used for formulation of a pharmaceutical agent . the pharmaceutical agent may be in any solid form of formulations such as powder , granule , tablet , troche , capsule , suppository , etc ., and may be used with practically all types of drugs .