Chloramphenicol's loss of favour began in the 1960s, when it was shown to have two distinct toxic effects on hematopoiesis: a frequently observed, dose-dependent anaemia, reversible on cessation of therapy, and an irreversible, 'idiosyncratic' aplastic anaemia, which had an incidence of approximately 1 case per 30,000 courses of therapy, a high case fatality rate, and no correlation with the or duration of treatment.