This study provides a critical perspective on "informant agreement (consensus) analysis" as it is used in ethnobotany and ethnopharmacology. It recasts the concept at a higher cultural level, and it describes the cultural agreement about reported medicinal plant use for the native peoples of North America. It examines some plant use categories around which there is significant cross cultural agreement, and some categories which lack such agreement. The study then proposes a theoretical approach to understanding the efficacy of plants lacking significant consensus in their usage. The study considers the implications of this second form of efficacy defined here as the "meaning response", but often referred to as the "placebo effect".