Creatine (CR) (methyl guanidine acetic acid), is recommended for application as food (dietary) supplement. It is supposed that human organism uses it for formation of CR phosphate, which is necessary as a source of energy for muscular work. The fate of supplemented CR is not as simple as that. Its molecule is decomposed and reused again in each recipient individually. In our short-term study we have observed changes in the metabolism of two-carbon (2C) units and of thiolic substances 4 to 6 hours after CR supplementation. The level of thiodiglycolic acid (TDGA) in urine increased rapidly and then decreased again to the original level. At the same time the pH values of urine increased by 1.5 units. The level of TDGA in urine is a marker of disturbance in metabolic pathways of 2C units and of thiolic compounds. Our long-term CR study, with quantitative evaluation of more than 30 parameters of potential metabolic products revealed that the biochemical pathways, affected by CR application included intrinsic participation of vitamin B-12 and folates. The levels of those two vitamins changed in a mutually reverse way. The men under study can be divided into 4 different groups according to changes of CR levels found in urine, and of levels of folates and vitamin B-12, determined in blood before and after CR administration. The probands of each group utilized CR as donor of 1C and 2C units derived from CR in different metabolic pathways. Differences between initial and terminal TDGA levels indicated that CR disturbed the equilibria of redox processes, catalyzed by folates and vitamin B-12. The levels of homocysteine increased in all but one probands. The amount of creatinine excreted into urine depended on the extent of metabolic disturbances. The observed changes indicated that exogenous application of CR affected metabolic pathways connected with endogenous synthesis as well as with decomposition of CR.