How to Order Prescriptions Description Doxycycline hyclate is a broad-spectrum antibiotic synthetically derived from oxytetracycline. To reduce the development of drug-resistant bacteria and maintain effectiveness of doxycycline hyclate and other antibacterial drugs, doxycycline hyclate should be used only to treat or prevent infections that are proven or strongly suspected to be caused by susceptible bacteria. Usage Doxycycline is an antibiotic used for treating bacterial infections including Potomac Horse Fever, other Ehrlichia infections, and Lyme disease. Doxycycline generally has similar activity to other forms of tetracyclines against susceptible organisms, but some strains of bacteria may be more susceptible to doxycycline. Important Note Intravenous injection of doxycycline has been associated with cardiac arrhythmia, collapse, and death in horses. See Doxy side effects under Product Label tab. Doxycycline is available in either a capsule or tablet, in various sizes. See above.