"Prevalence, Characteristics, and Publication of Discontinued Randomized Trials"
JAMA
http://jama.jamanetwork.com/article.aspx?articleid=1840235 
"Conclusions and Relevance  In this sample of trials based on RCT protocols from 6 research ethics committees, discontinuation was common, with poor recruitment being the most frequently reported reason. Greater efforts are needed to ensure the reporting of trial discontinuation to research ethics committees and the publication of results of discontinued trials."