A problem with studies that examine immobilization-related drug effects on large mammals is that no true control group exists because untreated (non-captured) animals in the same population have not been examined. We present data to show that overwinter survival in male bison (Bison bison) immobilized with carfentanil was the same as untreated bison. This unique experimental design allows us to conclude that the drug had no long-term effect on male bison overwinter survival.