Mylan NV and Pfizer Inc. are still struggling with manufacturing issues for the EpiPen anti-allergy injector as the critical back-to-school season gets underway and a rival generic product hits the market.

EpiPens, which can help stop a severe or life-threatening allergic reaction, have been hard to get since at least May 2018, when the U.S. Food and Drug Administration said the device was in short supply. Former and current employees of Pfizer’s Meridian Medical Technologies Inc., which makes the product for Mylan, said supply has been unsteady since at least 2017, when inspectors began noticing defects.

EpiPen allergy shots
Photographer: Daniel Acker/Bloomberg

Pfizer spokeswoman Amy...