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When Do Generics Challenge Drug Patents?


  • For helpful comments, we thank Tim Bresnahan, Al Engelberg, Joseph Farrell, Sherry Glied, Victor Goldberg, Bert Huang, David Hyman, Amy Kapczynski, Mark Lemley, Ed Morrison, Petra Moser, Matthew Neidell, Mitch Polinsky, Fiona Scott Morton, Howard Shelanski, Heidi Williams, two anonymous referees, and seminar audiences at Berkeley, Columbia, ETH Zurich, NYU, Rutgers, Toronto, Wisconsin, University of Delhi, Yale, the Federal Trade Commission, the Conference on Empirical Legal Studies, and the Pfizer Pharmaceutical Economics and Policy Conference. Annie Liang, Elizabeth Moulton, Tejas Narechania, and Nicholas Tilipman provided outstanding research assistance. We thank officials at the Food and Drug Administration, particularly Elizabeth Dickinson, Nancy Sager, and Joshua Sharfstein, for assistance with FDA approval data, including a FOIA request. IMS Health and the Lerner Center for Pharmaceutical Management Studies at Rutgers University furnished sales data. The Ford Foundation, Institute for Social and Economic Policy Research at Columbia University, and the Robert Wood Johnson Foundation's Public Health Law Research program provided financial support.

C. Scott Hemphill, Columbia Law School, Jerome Greene Hall, Rm. 727, New York, NY 10027; email: Hemphill is Professor of Law, Columbia Law School; Sampat is Assistant Professor, Department of Health Policy and Management, Mailman School of Public Health, Columbia University.


The Hatch-Waxman Act regulates competition between brand-name and generic drugs in the United States. We examine a feature of the Act that has attracted great controversy but little systematic attention. “Paragraph IV” challenges are a mechanism for generic drug makers to challenge the patents of brand-name drug makers as a means to secure early market entry. We first present descriptive results that chart the rise of brand-name patent portfolios and Paragraph IV challenges. Over time, patenting has increased, measured by the number of patents per drug and the length of the nominal patent term. Meanwhile, the fraction of drugs subjected to patent challenges has increased. Drugs are also challenged sooner, relative to brand-name approval. Our econometric analyses of challenges over the past decade show that brand-name sales have a positive effect on the likelihood of generic challenge. The likelihood of challenge also varies with the nature of the patent portfolio. A drug with weaker patents faces a significantly higher likelihood of challenge, conditional on sales and other drug characteristics. That is not because the drug's patent protection is weaker overall; additional patents, even weak ones, generally strengthen a brand-name firm's ability to exclude. Rather, a weak patent, particularly if it expires later than the basic patents, disproportionately attracts a challenge to the pertinent drug. Overall, our results suggest these challenges serve a useful purpose by promoting scrutiny of weaker and late-expiring patents.