More than 400 people in 45 states have informed advocacy group Food Allergy Research & Education (FARE) that they've had problems filling EpiPen prescriptions at their pharmacies, either having to wait several weeks or being unable to obtain the pens at all, according to Bloomberg.

Mylan, the company that distributes and is the rights holder of EpiPen knows that many people in the United States have had difficulties getting EpiPens, but they don't want to call it a shortage just yet. The Food and Drug Administration (FDA) thinks otherwise and has added generic EpiPens and other epinephrine auto-injectors to its drug shortage list.

This comes a month after Mylan said there was a shortage of EpiPens in Canada and the United Kingdom.

The FDA posted an update on Wednesday saying that while EpiPen remains available, there are "intermittent supply constraints" due to manufacturing delays.

Not everyone agrees on the scope of the problem.

"There's a problem out there. The FDA may not be fully aware of it because the companies that manufacture and distribute the EpiPen are not saying anything," FARE Chief Executive James Baker told Reuters. "We've got people who literally can't fill their prescriptions."

EpiPen The manufacturer of EpiPens faced criticisms regarding its ability to make functional EpiPens in 2017. (Photo: Amy Kerkemeyer/Shutterstock)

Mylan informed the FDA a few months ago that the company was experiencing "intermittent supply constraints" due to manufacturing challenges involving Pfizer's Meridian Medical Technologies division, which makes the auto-injecting pens for Mylan.

Meridian Medical had a number of high-profile incidents in 2017 regarding its ability to safely manufacture EpiPens. In March of that year, Meridian issued a voluntary recall of thousands of devices following complaints that the pens failed to activate.

In September, the FDA issued a warning letter to Meridian Medical, faulting them for failing to investigate reported product failures between 2014 and 2017, including not taking apart EpiPens to determine where the defects were in the devices.

A new system was put in place following the FDA's letter, and it's cited as the reason for the current lack of EpiPen access. Steven Danehy, a spokesman for Pfizer, told Bloomberg in a statement that the new processes have had "some impact on manufacturing capacity," but he also noted that EpiPen shipments have increased in recent months and were higher than expected in April.