BACKGROUND:The inclusion of patent linkage mechanisms in bilateral and plurilateral trade and investment agreements has emerged as a key element in the United States' TRIPS-Plus intellectual property (IP) negotiating agenda. However, the provisions establishing patent linkage mechanisms in several agreements appear to reflect a degree of ambiguity, potentially enabling some flexibility in their implementation. In this study, we reviewed the features of the prototypic patent linkage mechanism established by the Hatch-Waxman Act in the United States, and compared these with the implementation of systems in three countries whose agreements with the US include patent linkage obligations. From these analyses, we draw lessons for moderating the impact of these mechanisms on access to generic medicines. METHODS:We reviewed the features of the patent linkage mechanism in the US, and undertook a detailed analysis of relevant treaty provisions and the manner of implementation in Canada, Australia, and South Korea. RESULTS:A key difference between the US implementation of patent linkage and that of its trading partners is the disparate treatment afforded to biologics. Because of the significant differences in the regulatory frameworks applying to small molecule and biologic medicines in the US, the Hatch- Waxman provisions do not apply to biologics and they are not subject to patent linkage. By contrast, the regulatory frameworks in Canada, Australia and South Korea do not reflect similar distinctions and thus patent linkage mechanisms also capture biologics. Additional variations in implementation, mainly the result of constructive ambiguities in the respective treaty texts, offer potential opportunity for mitigating the adverse impact of patent linkage provisions on market entry of generic medicines. Practical measures include ensuring the availability of an accessible, transparent and easily searchable database of patent information; avoiding automatic stays of generic marketing approval where possible; and requiring certification by rights holders to prevent abuse of the system. CONCLUSIONS:Where countries accept treaty obligations to establish patent linkage mechanisms, the impact on access to generic medicines may be moderated to a degree by retaining and exploiting constructive ambiguities in the treaty text and addressing practical aspects of implementation.