The purpose of this study was to determine whether a peer nursing student who presents a longitudinal case study on warfarin in a pharmacology course classroom influences prelicensure and postbaccalaureate nursing students' knowledge and perceived knowledge about warfarin. The study was a descriptive design that used a convenience sample of baccalaureate nursing students enrolled in two pharmacology courses. All participating students answered warfarin case-study questions and completed a self-demographic questionnaire, a knowledge pretest and posttest, and a self-efficacy questionnaire after the activity, which evaluated students' knowledge and perceived knowledge on 11 warfarin concepts. For all students (N = 89), the number of correct answers improved significantly between pretests and posttests for Items 2-11 (p < .0001; Wilcoxon signed-rank tests), which evaluated students' knowledge on warfarin's site of action, associated laboratory values, use of vitamin K, and food-drug interactions. However, no significant difference was seen in the number of correct answers for warfarin's mechanism of action. Comparing prelicensure and postbaccalaureate groups by Mann-Whitney tests, no significant difference was seen for pretest total scores (median 7.00, n = 55; median 7.50, n = 34; respectively; p = .399). Similarly, no difference was seen for posttest total scores by groups (prelicensure: median = 9.00, n =54; postbaccalaureate: median = 10.00, n = 32; p = .344). Overall, students in both groups agreed that they could identify and explain all 11 warfarin concepts. The Pearson correlation between the total posttest and total self-efficacy scores for the combined group was .338 (p = .003), demonstrating a low but significant correlation between students' posttest total scores and their perceived warfarin knowledge, as evaluated by the self-efficacy questionnaire.