The US Food and Drug Administration(FDA) has sent a damning warning letter to drug giant Pfizer Inc (NYSE:PFE) after it received “hundreds of complaints” that the anti-allergy EpiPen device misfired “during life-threatening emergencies”.

The letter was sent to Meridian Medical technologies, the division of Pfizer that makes EpiPens for fellow pharma behemoth Mylan (NASDAQ:MYL).

It claimed that in some cases people had died after the device, which delivers a dose of epinephrine to people suffering from severe allergic reaction known as anaphylaxis, failed to work properly.

During an inspection at Meridian’s Missouri facility earlier this year, the FDA said it found multiple “significant” regulation violations related to the manufacturing of the device.

It is the same facility that produced a batch of EpiPens that was subject to a recall earlier this year after two reports of the auto-injectors misfiring.

“You failed to thoroughly investigate multiple serious component and product failures for your EpiPen products, including failures associated with patient deaths and severe illness,” read the warning letter.

“You also failed to expand the scope of your investigations into these serious and life-threatening failures or take appropriate corrective actions, until FDA's inspection.”

It concluded: “Correct the violations cited in this letter promptly. Failure to promptly correct these violations may result in legal action without further notice including, without limitation, seizure and injunction.”

In a statement given to CNBC, Pfizer said: “We currently have no information to indicate that there was any causal connection between these product complaints and any patient deaths.

"We are very confident in the safety and efficacy of EpiPen products being produced at the site.”

Pfizer shares were broadly flat in pre-market trading at US$33.98, while Mylan’s stock was down 0.4% to US$31.91.