Objective. to evaluate the clinical effect of a probiotic mouthwash in reducing generalized marginal chronic gingivitis using positive and negative control groups. methodology. four-week study conducted in San Luis Potosí, Mexico, from january to march 2017. participants were healthy, non-smokers with generalized marginal chronic gingivitis; age range 18-45 years.

subjects were randomized and divided into three groups:

group A: mouthwash based on 0.05 percent cetylpyridinium chloride (CPC) (positive control); group B: mouthwash based on probiotics (experimental); group C: placebo mouthwash (negative control). no oral hygiene practices or routines were modified; subjects were followed for 4 weeks. the primary outcome variable of interest was the Löe and Silness gingival index, and the secondary one, the Quigley Heinn plaque index modified by Turesky. results. of the 45 patients included, 19 (42.2 percent) were men and 26 (57.7 percent) women, mean age was 22.8±2.07. each group consisted of 15 subjects; all subjects completed the study. there was no statistically significant reduction in gingival inflammation when comparing the 3 treatment groups (p=0.540) with respect to the gingival index. A comparison was made before and after the treatment and in the 3 groups there was no reduction of the gingival inflammation. plaque reduction was not statistically significant when comparing the 3 groups (p=0.278). however, when doing intra-group comparison, it was found that the patients in group A had a reduction in plaque index (p<0.005), which was not observed in groups B (p=0.1103) and C (p=0.1508). conclusions. the use of a probiotic mouth mouthwash did not reduce gingival inflammation or the accumulation of dentobacterial plaque in a period of 4 weeks. there were no statistically significant differences between the study groups.