(CNN) EpiPen products can still deliver an effective dose long after they have expired, according to a study published Monday .

The active ingredient in auto-injectors like the EpiPen is epinephrine, more commonly known as adrenaline, a hormone that can help relax muscles. It can open airways and reduce swelling during a severe allergic reaction.

The price of the auto-injection devices has gone up 400% since 2007, drawing the ire of patients who rely on them to quickly counter life-threatening reactions. Outrage grew as more insurance providers dropped coverage of the EpiPen.

Amid the criticism, manufacturer Mylan developed a cheaper generic alternative priced at $300 for two pens.

With these patients in mind, researchers tested devices whose expiration dates had passed to determine whether they were still potent and safe.

Study author Lee Cantrell, professor of medicine and pharmacy at the University of California, San Diego, and his team measured the epinephrine concentration of 40 expired EpiPens and EpiPen Jrs. They found that 29 months after expiration, the pens contained at least 90% of their stated amount of epinephrine. Pens 50 months -- more than four years -- past the printed expiration date had more than 84% of the medication.

"Essentially, all of these would still be in the recommended therapeutic range," said Dr. Thomas Casale, a professor of medicine at the University of South Florida and executive vice president of the American Academy of Allergy, Asthma, and Immunology.

The new study builds on previous research finding that epinephrine auto-injectors have a much longer shelf life than labels said. A 2015 study found that some retained more than 90% of their stated dose two years after expiring.

Despite the findings, Casale, who was not involved in the new study, said patients probably won't want to take a chance with a potentially life-saving medication.

"Until those labels are changed, patients are in a bad situation, because they're concerned about having a life-threatening event and not having a medicine that's effective," he said.

Both Cantrell and Casale say that if someone has an allergic reaction, expired epinephrine is better than nothing at all.

"Our paper is not suggesting that people take expired medication," Cantrell said. "We can't make that leap based on our data. But what we can say is that if you have nothing else, and you're having a life-threatening reaction, certainly use the expired epinephrine."

Life-saving drugs

"The most common things we would prescribe epinephrine for is food allergies, bees, wasps and other stinging insects," Casale said. "The advantage of these auto-injectors is that you don't have to think about it. You just pop the cap, stick it right in your thigh, the needle comes out at the right depth into the muscle, and it delivers the correct amount of epinephrine."

In the study, all tested auto-injectors were within the therapeutic range for an effective dose. Simply using the amount of time since expiration is not always a good way to determine whether an EpiPen is still potent, and in the absence of testing equipment, the color of the solution might give some clues, Cantrell said.

Every pen has a small window where the liquid medication can be seen. It's usually clear and colorless. The quickest and easiest indication that the drug has expired is a color change to brownish-yellow, Cantrell said. "There's a little window on each one. If it's discolored, you shouldn't use them, period," he warned.

Good for backup

But what if your medication is past the printed expiration date and the color has not changed? "Personally, I think it's fine. I'm not convinced it becomes dangerous," Cantrell said.

Merilee Hufnagel, a mother of two in Johns Creek, Georgia, has kept her daughter's old EpiPens and says the new study validates her belief that they can still be good for a few years. Her 17-year-old daughter has dealt with life-threatening peanut allergies since she was a toddler.

"We've been buying them for 14 years. We started with the EpiPen Jr when our daughter was 3, and once she hit 60 pounds or so, the doctor switched her to the regular EpiPen," she said.

Luckily, the Hufnagels have not had to use their auto-injector pens, but they follow the company's guidelines and replace the devices each year.

Merilee Hufnagel keeps her family's expired EpiPens as backups.

However, Hufnagel says she knew that her expired pens were still good and kept them around just in case. "I keep other ones as backups because I know in my brain that they're still fine," she said.

"The reason I buy new ones? I want to be a good mom and follow the recommendations. That, and the schools will not accept them if they are expired," she said.

Although some schools purchase EpiPens, most still require students to provide their own and register them with the nurse's office. As with all medications, school policy requires families to update prescriptions and replace them when they are past their expiration date.

Replacing the pens can be costly. The prescription used to be covered by Hufnagel's insurance, and an online co-pay card from the company brought the total cost to zero. But the family's insurance provider recently stopped covering the name-brand device. To avoid paying $600 out of pocket, they opt for the cheaper generic device.

Mylan, which also makes the generic version, offers co-pay cards that brought the family's price down to $10 for two pens. Any additional pens needed would be purchased at cost: $300 each.

'Safe and effective'

Since the price controversy began, the shelf life of the auto-injectors has been studied by chemists and manufacturers alike. When exposed to light, heat or air, the epinephrine can degrade and lose its effectiveness.

In an emailed statement, Mylan said that the expiration date is the last day the medication is "safe and effective."

"All epinephrine products have expiration dating whether they come in an auto-injector or vial," the statement said. "This is required by law. An expiration date is the final day, based on performed quality control tests, that a product has been determined to be safe and effective when stored under the conditions stated in the package insert.

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"Given the life-threatening nature of anaphylaxis, patients are encouraged to refill their EpiPen Auto-Injector upon expiration, approximately every 12 to 18 months. Mylan also continues to invest in product improvements, such as a formulation with a longer shelf life."

Doctors and patients rely on information from manufacturers for the best information about the safety and efficacy of their medications.

Most patients like the Hufnagels religiously follow the manufacturer's guidelines, even amid growing evidence about the devices' potency.

"As far as adverse reactions to medication beyond expiration date, I haven't see any published study that shows it's truly a danger," Cantrell said.