[METHODS] A double-blinded , placebo-controlled , randomized clinical trial was conducted in 30 severe CP patients ( 14 males and 16 females , 40.5 8.4 years old on average from 35 to 60 ) receiving 3 different sequences of debridement-antibiotictherapy : Group A , antibiotic usage ( metronidazole , MTZ , 0.2 g , tid , 7 d ; amoxicillin , AMX 0.5 g , tid , 7 d ) was started together with SRP ( completed by 2 times in 7 d ) ; Group B , antibiotic usage ( MTZ 0.2 g , tid , 7 d ; AMX 0.5 g , tid , 7 d ) was started 1 d after SRP ( completed by 2 times in 7 d ) ; Group C , SRP alone [ probing depth ( PD ) , bleeding index ( BI ) and tooth mobility ] was examined .