Irritation, bioavailability, and residue aspects of ten oxytetracycline formulations administered intramuscularly to pigs.

After intramuscular administration to pigs (dosage 20-35 mg/kg) great differences were found among 10 injectable oxytetracycline (OTC) formulations with respect to irritation and persistence at the injection site and to the residue state of edible tissues. The OTC recovery percentages at the injection site 7-10 days after injection ranged from less than 0.001 to 0.15 per cent for three 10 per cent-formulations and from 0.12 to 20.5 per cent for seven 20 per cent-formulations. The OTC recovery percentage was obvious related to the extent of tissue irritation at the injection site, which was particularly evident for the 20 per cent-formulations. Serious doubt is expressed about advantages of so-called 'long-acting' formulations.