Florfenicol, as a synthetically produced monofluoro derivative of thiamphenicol, belongs to a new generation of animal-specific chloramphenicol-class (also known as amphenicol-class) broad-spectrum antibiotics. It does not carry the potential risk of causing aplastic anemia, making it relatively safe for clinical use. With the ban on chloramphenicol due to its severe suppression of bone marrow hematopoietic function and immunosuppressive effects, florfenicol has become its primary replacement, playing a crucial role in veterinary clinical practice.

Physicochemical Properties:
Florfenicol appears as a white or off-white crystalline powder, odorless. It is highly soluble in dimethylformamide, soluble in methanol, slightly soluble in glacial acetic acid, and practically insoluble in water and chloroform. The pH of its 5% aqueous solution ranges from 5 to 5.

Mechanism of Action:
Florfenicol enters bacterial cells via its lipophilic properties, primarily acting on the 50S subunit of the bacterial 60S ribosome. It inhibits transpeptidase and peptide chain formation, thereby exerting its antibacterial effect. Additionally, it selectively targets the 70S ribosomal receptors of bacteria, altering and interfering with peptidyl transferase function, further suppressing protein synthesis.

Pharmacokinetics:
After oral administration, florfenicol is rapidly absorbed, reaching therapeutic blood concentrations within approximately 1 hour and peak plasma concentrations within 1–3 hours, with a bioavailability exceeding 80%. Its intramuscular half-life is 2 hours, and it distributes widely in the body, achieving high concentrations in blood and bile while crossing the blood-brain barrier. The drug is primarily excreted unchanged in urine, with a small portion eliminated in feces. Under fed conditions, absorption remains nearly complete, with high oral bioavailability and evidence of enterohepatic recirculation.

Clinical Applications:
Florfenicol is widely used to treat various infections, secondary infections, and complications caused by multiple etiologies. It is particularly effective against respiratory diseases and exhibits potent antibacterial activity against most Gram-positive and Gram-negative bacteria, especially showing high sensitivity to Pasteurella and Actinobacillus pleuropneumoniae. Thus, it has become the drug of choice for these common swine diseases. Moreover, florfenicol demonstrates therapeutic efficacy against bacterial infections resistant to fluoroquinolones and other antimicrobials.

Precautions for Use:
Florfenicol forms insoluble complexes with metal cations and should not be mixed with such substances. Cross-resistance exists between florfenicol and thiamphenicol, warranting cautious use. Additionally, as florfenicol exhibits stronger immunosuppressive effects than chloramphenicol, it should be avoided or used sparingly during vaccination periods or in cases of severe immune compromise. Florfenicol also possesses embryotoxicity, necessitating caution in pregnant and lactating sows. High doses may induce peripheral circulatory failure in piglets, requiring strict dosage control. Prolonged oral administration may lead to digestive dysfunction and vitamin deficiencies; thus, concurrent multivitamin supplementation is recommended in swine herds.

Dosage:
The typical dosage ranges from 20–30 mg per kg body weight (calculated as florfenicol). Common formulations include 30% florfenicol injection and 50% florfenicol premix. For swine, the most accurate method is weight-based dosing. For example, a 100 kg finishing pig requires:

• 30% florfenicol injection (IM): 6 mL (minimum) to 10 mL (maximum)

• 50% florfenicol premix (feed additive): 4 g (minimum) to 10 g (maximum)
  Note: Due to florfenicol’s toxicity, ensure thorough feed mixing, and avoid water medication.

Adverse Effects:
Chloramphenicol carries a severe risk of aplastic anemia, attributed to the para-nitro group on its aromatic ring. In contrast, florfenicol’s structural modification—replacing the nitro group with a methylsulfonyl group—eliminates this risk. However, dose-dependent reversible bone marrow suppression may still occur, manifesting as anemia (due to inhibited erythropoiesis) or pallor (from leukopenia/thrombocytopenia).

Rational Use Considerations:
Despite florfenicol’s superior pharmacodynamics and relatively low toxicity among amphenicols, its widespread clinical use and frequent high-dose administration have contributed to resistant strains. Given increasing national focus on food safety, establishing scientifically sound dosing regimens—ensuring appropriate doses and intervals—is critical, particularly for this swine farm staple antibiotic.