AbstractThe aim of this study was to evaluate the effect of 3 mouthrinses, Listerine Antiseptic®(thymol), Peridex®(chlorhexidine), Perimed® (povidone iodine and hydrogen peroxide), and a placebo (water) on the development of dental plaque and gingivitis, when used as the only oral hygiene procedure for 14 days. 71 subjects were entered into a randomized, double‐blind study. At the baseline examination, papillary bleeding score (PBS), and plaque index (PI) were registered, after which subjects received supragingival prophylaxis and were assigned to 1 of 4 study cells. Subjects were asked to refrain from all oral hygiene procedures except for the supervised 14day 2 × daily rinsing with the assigned preparation. At day 14, the same clinical parameters were again registered. Statistical analysis was performed by a one‐way analysis of variance (ANOVA) to compare the 4 groups, followed by Duncan's multiple range test to determine specific group differences. At baseline, average PBS and PI scores were similar for all 4 groups. After 14 days, the average PBS for Peridex® and Perimed® was significantly lower than for Listerine Antiseptic® and water. The frequency of interdental units with a PBS>2 was significantly lower for Peridexl® and Perimed® than for Listerine Antiseptic® and water. We concluded that both Peridex® and Perimed® were effective in reducing plaque and gingivitis when used as a 2 × daily mouthrinse by subjects refraining from other oral hygiene procedures.In vitro, a synergistic effect was assumed when inhibition was achieved with Perimed® at the same or greater dilution than was achieved with povi