Nonideal enrollment of nonbiology majors into biology majors courses serves as an impediment to academic success and negatively impacts rates of college course completion. In this Institutional Review Board (IRB)-approved investigation, we examine student success, as measured by student grades and course completion rates, for nonbiology majors who nonideally enroll in biology majors and allied health courses. We compare their achievements with biology majors and allied health students enrolled in courses specifically designed for biology majors and those who pursue allied health paths, and with nonbiology majors who appropriately take biology courses designed for nonmajors. We show that under these circumstances, nonmajors perform worse in majors and allied health courses compared to their peers who enroll in the correct courses. In addition, our study shows that nonmajors have significantly higher attrition rates when enrolled in biology majors and allied health courses. Strategies to guide students and prevent nonideal course registration are discussed in the context of the Achieving the Dream (AtD) initiative, which uses a data-driven approach.