For this year’s flu season, many pharmacies are for the first time offering painless, needle-free shots to ward off the disease. But there’s one big problem: The FDA says they may not work.

The agency on Friday warned doctors, nurses and pharmacists “not to use injector devices to administer influenza vaccines.” The agency’s alert notes it has not approved any “injector devices” for use with the flu vaccine.

“The FDA has no data to support the safety or effectiveness” of flu vaccines delivered by jet injector, the alert reads.

So patients who already received the shot may need to go back and get a jab in the arm with a needle after all.

PharmaJet, a Colorado-based company, has been marketing its device as a proven flu-fighting tool.  In place of a needle, PharmaJet uses a high-pressure steam of liquid to pierce the skin and deliver the vaccine.

“PharmaJet’s needle-free injectors have received FDA marketing clearance for use with liquid medicines and vaccines and are available throughout the United States,” company president James Bowman said in a recent press release.

While the device has been approved for use with some drugs, FDA spokeswoman Shelley Burgess said flu vaccines are out of the question.

“Vaccines are labeled with specific instructions for how those vaccines should be administered,” Burgess said. “At this time, there are no vaccines for the prevention of influenza disease that are approved by the FDA for administration by jet injector.”

Heather Potters, chairman of PharmaJet, told ABC News company representatives have contacted top FDA officials to protest the move. Potters said there is nothing in FDA policy that should forbid medical professionals from using the PharmaJet injection device for flu vaccination and she insists jet injectors have been used for decades to successfully vaccinate people against many diseases, including influenza. The agency’s action, Potters said, came without warning.

Kroger Pharmacy, the fifth largest pharmacy chain in the United States, had been offering the PharmaJet injection to customers at some of its locations. On Friday, Kroger announced it had “immediately discontinued use of such clinical devices” at its pharmacies.

“Kroger is seeking guidance from the Centers for Disease Control and Food and Drug Administration regarding revaccinations,” a prepared statement released by the company said.

Bioject, an Oregon company, also makes a needle-free device that’s being used by several pharmacies to administer flu shots this year.  Publix, one of the nation’s largest grocery chains, had announced it would test the Bioject jet injection in 52 of its stores in the Southeast.

As for those who still need to get vaccinated, if you truly hate needles, there’s still the nasal spray FluMist.