The U.S. Food and Drug Administration (FDA) has OK’d a label change for Convenia (cefovecin sodium), extending the in-use shelf life of reconstituted vials of the antibiotic from 28 to 56 days.

Pfizer, maker of the single-injection cephalosporin designed to treat common canine and feline bacterial skin infections, plans to soon start notifying veterinarians of the change, approved by the FDA on Dec. 5. A news release is expected to go out Tuesday. 

The antibiotic’s formulation hasn’t changed, negating the need for regulators to reevaluate the drug’s safety and effectiveness data. However, in-use stability studies were conducted, said Laura Alvey, a spokeswoman for the FDA’s Center for Veterinary Medicine. 

Convenia hit the veterinary market in 2008, as the only antibiotic of its kind to be delivered via a single injection. The antibacterial effects of Convenia last approximately one to two weeks. Many veterinarians use it when treating feline patients in particular, because cats can be difficult when it comes to administering antibiotics in liquid or pill forms.

With just a 28-day shelf life, practitioners often discard reconstituted vials of Convenia when they haven't used the full 10-ml bottle before its expiration date.

At roughly $241 a bottle plus tax, the losses can get expensive, said Dr. Steven Barta, a practitioner in Cottrellville, Mich. Like many of his colleagues, he extends Convenia’s shelf life by splitting it up and freezing it. 

“It’s off-label, but that’s how we make it work. It’s what we’ve had to do to get by and still be able to use this drug,” Barta said in an interview with the VIN News Service. “It’s good for about 20 cat injections.” 

Barta explained that he keeps one aliquot of Convenia on hand and divvies up the rest among sterile vaccine bottles that he stores in a freezer. 

“I get by using it for several months at a time. Even with the label change, I’ll probably continue what I’m doing just because it’s been working.”

Pfizer does not recommend freezing Convenia, though the company has not stated whether its stance reflects efficacy or stability concerns with the drug. Discussions on the Veterinary Information Network (VIN) reveal that freezing Convenia is not unusual among practitioners. Pfizer officials did not respond when asked why the drug maker does not package Convenia in smaller aliquots that would allow practitioners to use the full bottle before it expires.

Dr. Wes Borgman, a practitioner near Orlando, Fla., is excited about the label change. "It's a good drug," he said. "With the label change, I think more veterinarians will use it."

He added that "most clients are very happy to not have to medicate their pets; they like this delivery method, and it increases compliance." Convenia is administered as a one-time shot while most other antibiotics commonly prescribed by small animal veterinarians come in liquid suspension or pill forms that owners must feed their pets daily.

Convenia's ease of use combined with practitioners' natural inclination to not waste half-filled vials has some worrying that the drug is being prescribed as a first-line antibiotic more often than needed.  

Dr. Michele Gaspar, a boarded feline practitioner and VIN consultant, an online professional community and parent of the VIN News Service, suspects that Convenia is being overused.

“Instead of being convenient for clients, this drug is being used like it’s convenient for colleagues,” Gaspar said. She wonders if veterinarians often turn to the drug because they want to go through a bottle of Convenia rather than wasting it. 

“One would hope that its expense and short shelf life would not be the reason we use this medication,” she said. 

Dr. Bill Folger, a boarded feline practitioner, VIN consultant and owner of Memorial Cat Hospital in Houston, shares Gaspar’s concerns. 

“I think people are using (Convenia) for all kinds of things that it was never designed for,” he said. While Convenia is labeled to treat skin infections, some veterinarians report using it after dental procedures and to treat urinary tract infections.

Folger isn't critical of using drugs off-label, but he thinks Convenia should be used conservatively.

"I don't go by labels; two-thirds of the drugs in my pharmacy are human-labeled pharmaceuticals. But when things sound too good to be true, they usually are," he said.