Between 2006 and 2010, six population based case-control studies were conducted as part of the European research-project DRUID (DRiving Under the Influence of Drugs, Alcohol and Medicines). The aim of these case-control studies was to calculate odds ratios indicating the relative risk of serious injury in car crashes. The calculated odds ratios in these studies showed large variations, despite the use of uniform guidelines for the study designs. The main objective of the present article is to provide insight into the potential effect of survey errors on the observed odds ratios for the six DRUID case-control studies. Relevant information was gathered from the DRUID-reports for eleven indicators for survey errors. The results showed that differences between the odds ratios in the DRUID case-control studies may indeed be (partially) explained by survey errors. Selection bias and random errors due to small sample sizes and cell counts were the most frequently observed survey errors in the six DRUID case-control studies. Therefore, it is recommended that epidemiological studies that assess the risk of psychoactive substances in traffic pay specific attention to describe these potential sources of bias. The list of indicators that was identified in this study is useful both as guidance for review studies and as checklist for epidemiological studies in the field of driving under the influence in order to decrease the effect of potential sources of bias already at the start of the study.