Published June 21, 2026 · Last updated July 29, 2026
How Convenia Works and Why It Lasts So Long
The single injection of Convenia (cefovecin sodium) is dosed at 8 mg/kg, given subcutaneously, and is FDA-approved for the treatment of certain skin infections (wounds and abscesses) and urinary tract infections in cats caused by susceptible organisms (FDA label, DailyMed). It belongs to the cephalosporin class, the same broad family as oral cefadroxil.
The "weeks-long" reputation is by design, and the pharmacokinetics explain why. According to the peer-reviewed study by Stegemann et al. published in the Journal of Veterinary Pharmacology and Therapeutics, cefovecin reaches peak plasma concentration within 2 hours of subcutaneous injection, with a mean terminal elimination half-life of 166 ± 18 hours — roughly 6.9 days in cats (Stegemann et al., 2006, PubMed). The drug is heavily protein-bound in plasma (99.5–99.8%), yet the free fraction in tissue fluid stays above the MIC90 for Pasteurella multocida for at least 14 days after a single dose. That 14-day therapeutic window is the whole point of the product — and the central safety trade-off.
Because Convenia cannot be "un-shot" once injected, every adverse event has to be managed symptomatically while the drug clears on its own. Roughly 50% of a dose is excreted in urine and about 25% in feces over 21 days (Stegemann et al., 2006), so kidney function has a direct impact on how long the drug lingers in your cat.
Documented Side Effects (and How Common They Are)
The FDA-approved prescribing information for Convenia reports the following adverse reactions from the pivotal U.S. field study (n = 147 Convenia-treated cats vs. n = 144 active-control cats on oral cefadroxil):
| Adverse reaction | Convenia (n=147) | Active control (n=144) |
|---|
| Vomiting | 10 | 14 |
| Diarrhea | 7 | 26 |
| Anorexia / decreased appetite | 6 | 6 |
| Lethargy | 6 | 6 |
| Hyperactivity / acting strange | 1 | 1 |
| Inappropriate urination | 1 | 0 |
Source: Convenia prescribing information, FDA DailyMed.
Post-approval pharmacovigilance has additionally reported less common reactions:
- Injection-site reactions: alopecia (fur loss), scabs, necrosis, and erythema at the subcutaneous injection site.
- Facial edema: swelling of the muzzle or face, which can be an early warning sign of a hypersensitivity reaction.
- Hemolytic anemia (rare, but reported in post-approval surveillance).
- Pruritus (itching) and excessive salivation.
- Anaphylaxis — rare, but a true emergency.
If you notice facial swelling, hives, sudden collapse, pale gums, or rapid breathing after the injection, head to the nearest open veterinary clinic immediately. For a deeper look at warning signs, see our Convenia allergic reactions guide and our full Convenia side effects reference.
What to Ask Your Vet Before the Shot
In routine feline practice, Convenia is safest when a quick baseline check is done first. Here is what to expect — and what to ask for:
- Recent bloodwork. A basic chemistry panel flags pre-existing kidney or liver disease. Because Convenia is renally excreted, reduced kidney function means the drug lingers longer than the label assumes, which raises the risk of any reaction.
- A confirmed diagnosis. Convenia is specifically labeled for skin infections (wounds, abscesses) and urinary tract infections caused by susceptible organisms. It is not a one-size-fits-all antibiotic for fever of unknown origin. If your cat is straining in the litter box, our cat UTI guide covers what your vet should rule out first.
- A drug-allergy history. Any prior reaction to a penicillin or another cephalosporin is a red flag — cross-reactivity within the β-lactam family is real, and the Convenia label explicitly warns against it.
- A 48–72 hour observation window. Most mild GI and lethargy reactions show up in the first two to three days. Plan to keep your cat indoors, easy to monitor, and on the same food during this window so any appetite change is obvious.
When to Stick With Oral Meds Instead
- Your cat has a history of drug allergies (especially to penicillins or other cephalosporins).
- Your cat has advanced kidney disease — Convenia is renally excreted and the dose cannot be easily adjusted once injected.
- Your cat is a "pro" at taking pills — especially in a pill pocket — and oral meds will reliably get into them.
- The infection is complex and might require switching drugs mid-stream. With pills, you can stop and pivot; with a one-shot antibiotic, you cannot.
This guide reflects FDA-approved label data and peer-reviewed research, but no article — and no AI assistant — can substitute for a hands-on veterinary examination of your specific cat. If you suspect an adverse reaction, contact your veterinarian or an emergency clinic right away.
