In a crossover study , dogs were randomly assigned ( in groups of 10 ) to receive a single oral dose ( 20 mg/kg ) of doxycycline hyclate without excipients ( control ) or extended-release formulations ( ERFs ) containing doxycycline , acrylic acid polymer , and polymethacrylate in the following proportions : 1:0.5:0.0075 ( ERF1 ) or 1:1:0.015 ( ERF2 ) .