Inhalational anthrax (post-exposure): Adults: 100 mg, of doxycycline, by mouth, twice-a-day for 60 days. Children: weighing less than 100 lb (45 kg); 1 mg/lb (2.2 mg/kg) of body weight, by mouth, twice-a day for 60 days.Children weighing 100 lb or more should receive the adult dose.oxycycline is indicated for treatment of infections caused by the following gram-positive microorganisms when bacteriologic testing indicates appropriate susceptibility to the drug: Upper respiratory infections caused by Streptococcus pneumoniae.