Fixed orthodontic appliances (braces systems) contain variable amounts of nickel and chromium, which can lead to immune responses and allergic reactions. The objective of the present review was to evaluate the changes in the quantity of nickel and chromium in saliva among patients wearing braces systems. The electronic databases used to perform the search were Pubmed, ReasearchGate, and Google scholar. After an initial search of these electronic databases, 12 studies were included in the systematic literature review. Overall, 554 patients wearing non-removable orthodontic appliances participated in the research that we reviewed. The age of the subjects ranged from 11 to 35 years. The quantity of chromium and nickel ions in saliva was measured before the braces systems were placed and after some period of having them. After evaluating the changes in nickel and chromium levels in saliva during orthodontic treatment with fixed brackets, it can be concluded that there is an increase in the levels of nickel and chromium ions in saliva. There is a significant increase in these ions after placement of the orthodontic appliances, peaking from three to six months, but not reaching toxic levels. The levels then gradually decrease. A full and detailed examination of the patient before starting orthodontic treatment is important, and alternative orthodontic appliances for patients who are allergic to nickel and/or chromium should be recommended.