Using a randomized controlled design , 61 subjects with moderate to advanced periodontitis were assigned to three parallel treatment groups : 1 ) SRP combined with prescription-tray ( Perio Tray ) application of 1.7 % hydrogen peroxide gel ( Perio Gel ) and , for the first 2 weeks , doxycycline , 2 ) SRP combined with prescription-tray application of peroxide gel , and 3 ) SRP alone .