The objective of this review was to identify and critically analyze the evidence in the scientific literature, through clinical trials, to compare the effectiveness of the various concentrations of chlorhexidine with the same solution and the one most frequently used in the oral hygiene of hospitalized patients. The authors conducted an electronic and manual search in several databases (Pubmed, Scielo, Lilacs, Cochrane) and the gray literature (Thesis Bank and Google Scholar). There was no limitation as to the year of publication or language. A total of 176 articles were selected and systematically analyzed according to the pre-defined inclusion criteria (clinical trials, randomized or not, in any language, conducted in sick patients, regardless of gender, age, or race, admitted to hospitals, whether in the ICU or other sectors). These articles were evaluated, the data considered relevant were extracted, and a classification was made according to the quality of the methodological evidence found, with level I if it met all the criteria or four (with only one B). Level II if it partially met the criteria (maximum two C assessments). Level III if it followed two criteria or less (more than two C's). The only study selected was classified as level of evidence III, as it did not include the information necessary for a good quality study and, therefore, was considered to have a high risk of bias. Regarding the treatment used, the chlorhexidine solution was compared to the same product in different concentrations in the control group. Given the great variability in the concentrations of this mouthwash, further research should be carried out in order to achieve greater certainty and standardization of which concentration is shown to be more effective in the prevention of aspiration pneumonia in hospitalized patients.