ABSTRACTBackground: Orphan drugs (ODs) are pharmaceuticals manufactured for rare conditions that affect less than 200,000 people in the US. ODs are therefore produced in small quantities to meet sparse demand. Since 2010, OD shortages have become frequent, but no comprehensive, quantitative studies exist.Objective: The objective of this study is to assess the rates of OD shortages per therapeutic class and their trends over time in the United States.Study design: OD approvals were collected from publicly available information on the US Food and Drug Administration (FDA) website on 13 June 2016. Data on OD shortages were collected from the FDA and the American Society of Health-System Pharmacists (ASHP) websites. We reviewed the number of shortages per year and per therapeutic area. Multiple indications for the same drug were counted individually.Results: Of 569 ODs approved, 50% were approved in the decade ending in 2015. Oncology was found to be the most represented therapeutic area (34% of all OD approvals), followed by endocrinology (11%). Shortage data were available from 2008. In total, there were 66 (12%) OD shortages, with an average shortage duration of 455.5 days. Shortages were observed mainly for oncology products (19 cases, 13% of oncology ODs) and endocrinology products (14 cases, 22% of endocrinology ODs)Conclusion: Despite the FDA strategic plan for preventing and mitigating drug shortages (October 2013), remaining OD shortages still pose an enduring challenge to patient care, with a median shortage duration of almost 15 months. In many instances, ODs are the only available therapy for rare diseases, and OD shortages can lead to serious health deterioration and death. More research is needed to elucidate the causes of shortages and their impact on patients? health.