Letter to the editor: Re: The clinical effect of scaling and root planing and the concomitant administration of systemic amoxicillin and metronidazole: a systematic review; Re: Effectiveness of systemic amoxicillin/metronidazole as adjunctive therapy to scaling and root planing in the treatment of chronic periodontitis: a systematic review and meta-analysis; Re: Effectiveness of systemic amoxicillin/metronidazole as an adjunctive therapy to full-mouth scaling and root planing in the treatment of aggressive periodontitis: a systematic review and meta-analysis.