Doxycycline is an antibiotic in the Tetracycline family. Doxycycline is one of the most frequently used oral antibiotics for acne treatment. Doxycycline can be an effective treatment for many people with moderate to severe acne. Unfortunately, acne-causing bacteria that are resistant to Doxycycline (and other Tetracycline antibiotics) are becoming increasingly common. Because of this antibiotic resistance, doxycycline is ineffective for some patients. Doxycycline is available in oral and topical formulations, although oral use is the most common.