Anecdotes that provide definitive evidence

Many adverse drug reactions are first reported anecdotally. Anecdotal reports, by which we mean either individual cases or small case series, are generally regarded as providing poor quality evidence. They therefore usually require formal verification through robust epidemiological studies or clinical trials, although a minority are actually verified. However, we propose that some adverse drug reactions are so convincing, even without traditional chronological causal criteria such as challenge tests, that a well documented anecdotal report can provide convincing evidence of causal association and further verification is not needed. Such reactions could serve as gold standards for use, for example, when validating pharmacovigilance systems or assessing the quality of systematic reviews of adverse drug reactions and the methods used to perform them. Specificity of an adverse drug reaction has previously been discussed as a concept but to our knowledge has never been fully developed.