Two previous double-blind studies have shown that 1 week of metronidazole, plus scaling and root planing, can significantly reduce the number of teeth per patient that require periodontal surgery, when compared to a positive-control treatment that consisted of 1 week of placebo treatment plus scaling and root planing. The only difference between the two studies was that in one, metronidazole or placebo tablets were taken at the beginning of the debridement procedures, while in the other, the tablets were taken after all debridement was completed. This article describes the results obtained by combining the data from both studies and using an analysis of variance to identify the effects of medication and those of the study protocol. There was an average reduction in surgical needs of about 7.1 teeth per patient using metronidazole compared to 2.4 teeth in the positive-control group (P = 0.004). In the deep pockets, there was a significant protocol effect because there were additional improvements associated with using the medications after the debridement was completed.