Florfenicol is a synthetic broad-spectrum antibiotic effective against most Gram positive and Gram negative bacteria isolated from domestic animals.

Florfenicol acts by inhibiting bacteria protein synthesis at the ribosomal level, thus is bacteriostatic. However, in vitro tests have shown that florfenicol has a bactericidal activity against the most commonly isolated bacterial pathogens involved in respiratory diseases:

Acquired resistance to florfenicol is mediated by efflux pump resistance associated with a floR gene. Such resistance has not yet been identified in the target pathogens except for Pasteurella multocida. Cross resistance with chloramphenicol can occur. Resistance to florfenicol and other antimicrobials has been identified in the food-born pathogen Salmonella typhimurium and co-resistance with the third-generation cephalosporins has been observed in respiratory and digestive Escherichia coli.

For Mannheimia haemolytica, Pasteurella multocida and Histophilus somni the following breakpoints have been determined for florfenicol in bovine respiratory disease; susceptible: ≤2 µg/ml, intermediate: 4 µg/ml, resistant: ≥8 µg/ml.

In bovine, 99% of P. multocida isolates (n=156) and 98% of M. haemolytica isolates (n=109) were susceptible to florfenicol (strains isolated in France in 2012).

In swine, 99% of P. multocida isolates (n=150) were susceptible to florfenicol (strains isolated in France in 2012).

The following Minimum Inhibitory Concentrations (MIC) have been determined for florfenicol in European isolates collected from diseased animals between 2009 to 2012:

Bacteria species	Origin	Nb of strains	CMI of florfenicol (µg/mL)
			CMI50	CMI90
Mannheimia haemolytica	Cattle	147	0.7	1.0
Pasteurella multocida	Cattle	134	0.3	0.5
Histophilus somni	Cattle	64	0.2	0.2
Pasteurella multocida	Swine	151	0.4	0.5
Actinobacillus pleuropneumoniae	Swine	158	0.2	0.4

In cattle:

Intramuscular administration of the formulation at the recommended dose of 20 mg/kg maintains efficacious blood levels for 48 hours. Maximum mean serum concentration (Cmax) of 3.8 µg/ml occurred 5.7 hours (Tmax) after dosing. The mean serum concentration 24 hours after dosing was 1.95 µg/ml. The mean elimination half life was 15.3 hours

In pigs:

After intramuscular administration of florfenicol, maximum serum concentration of 4.7 µg/ml is reached after 1.8 hours and the concentrations deplete with a terminal mean half-life of 14.8 hours. Serum concentrations drop below 1 µg/ml, the MIC90 for the target porcine pathogens, 12-24 hours following IM administration. Florfenicol concentrations achieved in lung tissue reflect plasma concentration, with a lung: plasma concentration ratio of approximately 1. After administration to pigs by the intramuscular route, florfenicol is rapidly excreted, primarily in urine. The florfenicol is extensively metabolised.