Study: Single Exposure to the Heat of a Car Can Lower Effective Dose of Epinephrine Auto-Injector

A study conducted at the Medical University of Lodz, Poland confirms what you already knew: never leave your epinephrine auto-injector in your car on a hot day, because the drug degrades quickly and the device may not administer a sufficient dose when it’s needed.
Researchers purchased a dozen 0.3mg “adult dose” EpiPen brand auto-injectors from the same lot. They placed nine of the devices in the glove compartment, cabin, and trunk of a car parked in a treeless area and the remaining devices in a dark, air-conditioned room at the recommended storage temperature.

After half a day, the researchers retrieved the EpiPens stored in the car, cooled them to room temperature, then tested the contents of all the devices.
Their analysis determined that the epinephrine from the devices stored in the trunk had degraded by 3.3%, 13.3% for those from the cabin, and 14.3% for the devices from the glove compartment.
As the recommended dose of epinephrine for an adult suffering a reaction is 0.3mg, a single exposure can render the effective epinephrine in an auto-injector below recommendations.
The moral of the story: Don’t leave your auto-injector in a car or other hot place.
Here is Mylan’s recommendation for storage of EpiPen:

Storing the EpiPen® Auto-Injector and Its Authorized Generic

EpiPen® and EpiPen Jr® Auto-Injectors and Epinephrine Injection, USP should be stored in the carrier tube provided at a temperature of 20-25 °C (68-77 °F); however, temperature excursions between 15-30 ºC (59-86 ºF) are permitted. EpiPen®, EpiPen Jr®, and Epinephrine Injection, USP Auto-Injectors should not be stored in refrigerators or in a vehicle’s glove box. EpiPen® Auto-Injectors and their authorized generic auto-injectors should not be exposed to extreme heat or cold and should be protected from light.