Mix it up: Resuspension times for injectable antibiotics can vary widely

An analysis of three similar injectable antibiotics used for swine respiratory disease pathogens demonstrates that resuspension times can differ substantially — a difference that could lead to improper dosing, according to Ashley Johnson, DVM, technical service veterinarian, Zoetis.

If a bottle of injectable antibiotic isn’t shaken long enough to ensure the drug is properly resuspended, the result is very likely to be underdosing that could result in inadequate treatment, Johnson said.

Conversely, improper resuspension can also lead to an overdose when most of the bottle has been used and all that’s left is the active ingredient settled on the bottom. Overdosing can not only have adverse effects on the animal, it could lead to a residue violation, she cautioned.

The analysis Johnson cited compared resuspension times for Excenel® RTU EZ (ceftiofur hydrochloride), Excede® for Swine (ceftiofur crystalline free acid) and a generic version of Excenel® RTU ceftiofur hydrochloride ready-to-use sterile suspension. All three are indicated for some of the common pathogens involved in swine respiratory disease.

Zoetis scientist Kent Alleman, PhD, who led the analysis, said vials of each antibiotic were stored upright at room temperature for 10 days prior to testing. To test for resuspension, the vials were shaken by hand, and 5 mL samples were collected at regular intervals for up to 90 seconds. The samples were then tested for potency to determine the extent of resuspension. Multiple vials of each product were sampled, and each 5 mL sample was tested three times.

Resuspension recovery

“At the 10-second interval, we had at least 90% mean recovery with Excenel RTU EZ and Excede for Swine, but for the generic version of Excenel RTU, it was only about 50% resuspended,” he reported.

The generic product showed a gradual increase in resuspension over the 90-second study interval to a maximum of 87%, while Excenel RTU EZ and Excede for Swine remained suspended ≥ 88% through 50 seconds (Figure 1), Alleman said.

“That means that to ensure resuspension,” Alleman said, “the generic product would have to be shaken for at least 90 seconds.” This is consistent with the product label, which also instructs users to shake for 90 seconds to ensure complete resuspension.

Johnson said that anyone injecting swine, especially in large numbers, knows there may not be time to spend repeatedly shaking a bottle to make sure the contents are thoroughly mixed.

“Shaking for even half a minute can seem like an eternity,” she said. In addition, injectable antibiotics in a commercial swine setting need to be administered correctly for optimum effectiveness. “If the product is more convenient for caregivers to use, it’s more likely to be used correctly and the outcomes may be better.”

Johnson noted that Excede for Swine and Excenel RTU EZ are covered by the Zoetis Residue Free Guarantee. If one of these products is used according to the label but a violative residue occurs, Zoetis will compensate for the pork market value of the animal.

“Residue violations are most likely to occur when a drug isn’t administered exactly as it should be, which can be avoided with careful veterinary oversight,” she said. “One way veterinarians can prevent improper dosing is by educating caregivers about the importance of properly resuspending injectable antibiotics.”



People with known hypersensitivity to penicillin or cephalosporins should avoid exposure to EXCENEL RTU EZ. Do not use in swine found to be hypersensitive. Withdraw 6 days prior to slaughter when injection site volumes are greater than 5 mL up to 15 mL per injection site and 4 days prior to slaughter when injection site volumes are less than or equal to 5 mL per injection site. See full Prescribing Information on page 5 of this booklet.



People with known hypersensitivity to penicillin or cephalosporins should avoid exposure to EXCEDE. Do not use in swine found to be hypersensitive to the product. Pre-slaughter withdrawal time is 14 days following the last dose. See full Prescribing Information on page 6 of this booklet.


Click here to see full Prescribing Information for use in swine for Excenel RTU EZ.

Click here to see full Prescribing Information for Excede for Swine.



All trademarks are the property of Zoetis Services LLC or a related company or a licensor unless otherwise noted. 


Discoveries is a series of  research news reports written by the editors of Pig Health Today on behalf of the US Pork Business of Zoetis.

January 2021