GAO's review of Westat's evaluation reports and associated documentation leads to the conclusion that the evaluation provides credible evidence that the campaign was not effective in reducing youth drug use, either during the entire period of the campaign or during the period from 2002 to 2004 when the campaign was redirected and focused on marijuana use. By collecting longitudinal data--i.e., multiple observations on the same persons over time--using generally accepted and appropriate sampling and analytic techniques, and establishing reliable methods for measuring campaign exposure, Westat was able to produce credible evidence to support its findings about the relationship between exposure to campaign advertisements and both drug use and intermediate outcomes. Given that Westat's evaluation stated the campaign did not reduce youth drug use nationally, Congress should consider limiting appropriations for the campaign, beginning in the 2007 fiscal year budget until the Office of National Drug Control Policy (ONDCP) provides credible evidence of a media campaign approach that effectively prevents and curtails youth drug use. ONDCP's written comments on our report generally disagreed with the findings. Specifically, ONDCP does not believe the Westat findings reflect the campaign's effectiveness. GAO believes the Westat study is sound. Appended are: (1) Westat's Methods for Addressing Evaluation Implementation Issues; (2) Comments from the Office of National Drug Control Policy; and (3) GAO Contacts and Staff Acknowledgments. (Contains 2 tables, 1 figure, and 30 footnotes.) [This report was produced by the Government Accountability Office, formerly known as the General Accounting Office.]