The cause and prevention of recurrent aphthous stomatitis (also called aphthous ulcers or canker sores) are still unknown. This may be due in part to ignorance of the risk factors present in susceptible people. In this systematic review (PROSPERO record #CRD42019122214), we show that most of the risk factors for the disease are single nucleotide genetic polymorphisms in genes related to the functioning of immune system (TLR4, MMP9, E-selectin, IL-1 beta and TNF-alpha). Single nucleotide genetic polymorphisms do not constitute a modifiable risk. This indicates that, at least in part, susceptibility to recurrent aphthous stomatitis is hereditary, and that these factors cannot be modified.