This article shows an approach in evaluating and simultaneously furthering the implementation of a national warning campaign using participatory research. The campaign was a response to contaminated cocaine that appeared on the European drug market in 2004, causing extraordinary health risk for drug-users. To counter this, an elaborated warning campaign was conducted by the Drug Information and Monitoring System, a toxico-epidemiologic monitor of drug markets in the Netherlands. The process of this intervention was evaluated by a combination of qualitative and quantitative research methods to acquire valid and useful results. A concluding Delphi-technique resulted in shared proposals for improvement of future warning campaigns and the monitoring process. Amendments in the protocol for warning campaigns were made to facilitate the communication between different actors and to clarify responsibilities. Problems of information provision were tackled by the development of a reporting system of drug incidents. Further, the national coverage of the monitor was ensured by inviting new participants and the structure of the monitor was improved by setting up a monitor of drug incidents. The Delphi-technique also contributed to mutual understanding and common ground, and thus to optimising the conditions for further implementation.