Efficacy and safety concerns are important reasons why the FDA requires multiple reviews before approval of new drugs.

The regulatory approval of new drugs by the Food and Drug Administration (FDA) is a long and complex process and often requires multiple cycles of review, potentially delaying patients' access to new and effective therapeutics. We used qualitative methods to characterize the safety and efficacy reasons why applications for novel therapeutics approved by the FDA between 2001 and 2011 required multiple review cycles prior to approval. Among ninety-six applications approved between 2001 and 2011 that required multiple review cycles, safety concerns contributed to seventy-four (77.1 percent) and efficacy concerns to forty-three (44.8 percent). Our study suggests that multiple review cycles appear to play an important role in allowing the FDA to protect public health and in ensuring adequate understanding of clinical benefits and risks prior to approval.