The medicine in emergency allergy shots like the EpiPen can deteriorate when exposed to heat, researchers caution.

Patients with serious allergies typically carry an emergency autoinjector – such as the EpiPen made by drugmaker Mylan NV – at all times. The devices are used to deliver an emergency dose of the hormone epinephrine to patients who may go into life-threatening anaphylactic shock.

“We work with hundreds of patients with histories of anaphylaxis, who carry epinephrine on a daily basis,” lead author Piotr Lacwik, who works at the Medical University of Lodz in Poland, said in an email. “I noticed that not all of them have their epinephrine on them at all times and, alarmingly, some leave injectors in the car.”

For the study, Lacwik and his team purchased 12 EpiPen Senior injectors from the same lot to ensure consistency. They distributed nine EpiPens between the glove compartment, cabin shelf and trunk of a car parked in a treeless area. The remaining three were stored in a dark, air-conditioned room at a constant temperature.