It has been claimed that oral creatine supplementation might have potential cytotoxic effects on healthy consumers by increasing the production of methylamine and formaldehyde. Despite this allegation, there has been no scientific evidence obtained in humans to sustain or disprove such a detrimental effect of this widely used ergogenic substance.
Poortmans, J. R., Kumps, A., Duez, P., Fofonka, A., Carpentier, A., & Francaux, M. (2005). Effect of oral creatine supplementation on urinary methylamine, formaldehyde, and formate. Medicine and science in sports and exercise. https://hdl.handle.net/2078.5/209410