Used primarily as an antibiotic in veterinary medicine, especially in livestock such as cattle, swine, and poultry to treat bacterial infections including respiratory diseases, urinary tract infections, and certain gastrointestinal infections. Commonly administered through feed, water, or injection. Also applied in agriculture to promote growth and prevent disease in crowded farming conditions. In humans, it has been used historically for treating infections like acne, chlamydia, and rickettsial diseases, though its use is now limited due to resistance and the availability of newer tetracyclines. Occasionally used in research settings for gene expression systems that rely on tetracycline-controlled transcription.