20 consecutive adult periodontal patients, requiring cause-related therapy as phase 1 of their treatment plan, and presenting ≥4 sites with probing pocket depth ≥4mm will be enrolled. For each patients four sites will be considered for the study, and two sites will be allocated in the test group and two sites will be allocated in the control group. In the test group, ultrasonic debridement of the pocket will be performed using a piezoceramic ultrasonic device with the a tip connected to the handpiece for 5 minutes/pocket. Then, it will be followed by the subgingival use of erythritol powder 2x5 seconds/pocket. At this time the subgingival delivery of metronidazole gel will be provided. In the control group, the same protocol will be used except for the use of a placebo instead of metronidazole. After instrumentation, patients will rinse with chlorhexidine 0.20% 3 times/day for 2 weeks.