Tetracycline HCL is an antibiotic that fights infection caused by bacteria. Tetracycline is used to treat many different bacterial infections of the skin, intestines, respiratory tract, urinary tract, genitals, lymph nodes, and other body systems. It will not work for viral infections (such as common cold, flu). Using any antibiotic when it is not needed can cause it to not work for future infections. Dosage is based on the medical condition and response to therapy. Tetracycline HCL is very effective.