According to a recent study, 46.6% of all emergency medicine drug shortages ...

... shortages between January 2001 to March 2014 occurred for unknown reasons. From January 2008 to March 2014:

• emergency medicine drug shortages rose 435%, from 23 to 123
• shortages for drugs used as lifesaving interventions or for high-acuity conditions rose 393%, from 14 to 69
•  shortages for drugs with no available substitute rose 125%, from 4 to 9

Source: "Longitudinal Trends in U.S. Drug Shortages for Medications Used in Emergency Departments (2001–2014)," Academic Emergency Medicine, January 2016, http://onlinelibrary.wiley.com/doi/10.1111/acem.12838/full