A book published by Ben Goldacre this year has drawn attention to the fact that drug trials producing negative or nil results are simply not published, thereby making the drugs appear to be more successful than they really are. Goldacre is spearheading a movement to force drug companies into publishing all their results and ensuring that there is free access to such data. (Most research on the effectiveness of drug treatments is funded and controlled by the drug companies).
One example of the effects of such non-reporting was mentioned in an article by Irving Kirsch (here). The effectivness of drug treatments is often assessed by making comparisons between a drug-treatment group and a group of patients receiving a placebo. Meta-analyses of antidepressant medications have reported only modest benefits over placebo treatment. However, when unpublished trial data are included, the benefit falls to almost nothing.