A news story in Nature Medicine (14, 1133; 2008) discusses an investigation into the publication status of the clinical-trials literature (PLoS Med., doi: 10.1371/journal.pmed.0050191; 2008), which concludes that positive results of clinical trials for drugs

http://blogs.nature.com/peer-to-peer/2008/11/positive_skew_of_clinicaltr...

Publication is positively skewed. Bjorn G (2008) Nat Med.
Medicine. Moving toward transparency of clinical trials. Zarin DA (2008) Science.
Publication of clinical trials supporting successful new drug applications: a literature analysis. Lee K (2008) PLoS Med.
November 27, 2008