In the face of tight public budgets more and more studies are being funded by the pharmaceutical industry. At the same time responsibility for conducting company-funded trials is increasingly being shifted to contract research organisations. Pharmaceutical manufacturers sponsor trials that primarily pursue company interests. The dominance of company-funded research does not only have a bearing on the choice of study priorities, though. Company sponsorship also has an influence on the results of trials. Company-funded trials are four times more likely to find evidence in favour of the trial drug than studies funded by other sponsors. There are several contributory factors, from study design (design bias) to data manipulation. And non-publication (publication bias) can distort knowledge. As a result, it is largely impossible to reliably assess the benefit and harm of medical drugs on the basis of published trials. This will have repercussions for the reliability of meta-analyses, guidelines and patient information leaflets. One consequence may be treatment errors.