The Food and Drug Administration this week accused the drugmaker Pfizer of failing to properly investigate reports of malfunctioning EpiPens, including incidents when patients died or became severely ill after the device failed to work. Pfizer manufactures the EpiPen, which treats allergic reactions, for the drugmaker Mylan.

In a warning letter issued Tuesday, the agency said Meridian Medical Technologies, which is a unit of Pfizer, did not adequately look into problems with a crucial component of the EpiPen — the mechanism on the device that insures that it fires and delivers the proper dose of epinephrine, which stops an allergic reaction.

The F.D.A. said the company failed to conduct a proper investigation even though it received numerous complaints about problems with activating the device. “Our own data show that you received hundreds of complaints that your EpiPen products failed to operate during life-threatening emergencies, including some situations in which patients subsequently died,” the agency said in the letter.

A spokeswoman for Pfizer, Kim Bencker, said in a statement that the company was “very confident in the safety and efficacy of EpiPen products being produced at the site” and noted that it has shipped more than 30 million EpiPens since 2015. “It’s not unusual to receive product complaints, especially when the product is frequently administered by non-medically trained individuals.”