This paper assesses data quality in the first two rounds of the European Web Survey on Drugs (EWSD). Data quality has various dimensions, such as accuracy, consistency, validity and completeness, but can be defined as fitness of the data for the intended use. For the quantitative assessment, the paper considers issues such as respondent fatigue and impact of the item format on responses. In conjunction, the authors conducted a qualitative data quality assessment, within a broader project that aimed to assess the validity and reliability of the Czech version of the questionnaire used in the first round of the EWSD. The paper shows how these assessments were used to improve the design of the questionnaire used in the second round of the EWSD, which appear to have yielded more accurate responses to the survey. This underscores the value of conducting data quality assessments on an ongoing basis to ensure that the questionnaire continues to reflect current drug use practices, which are fluid and may rapidly change. Overall, the paper provides insight into the validity of the EWSD, while also underscoring the importance of data quality assessments in future web survey data collection on drug use and related behaviours.