 For neurosyphilis , due to the poor penetration of penicillin G into the central nervous system , those affected are recommended to be given large doses of intravenous penicillin for a minimum of 10 days . If a person is allergic , ceftriaxone may be used or penicillin desensitization attempted . Other late presentations may be treated with once @-@ weekly intramuscular penicillin G for three weeks . If allergic , as in the case of early disease , doxycycline or tetracycline may be used , albeit for a longer duration . Treatment at this stage limits further progression but has only slight effect on damage which has already occurred . 
